TOEFL综合写作练习

写文书 In the United States, employees typically work five days a week for eight hours each day. However, many employees want to work a four-day week and are willing to accept less pay in order to do so. A mandatory policy requiring companies to offer their employees the option of working a four-day workweek for four-fifths (80 percent) of their normal pay would benefit the economy as a whole as well as the individual companies and the employees who decided to take the option. The shortened workweek would increase company profits because employees would feel more rested and alert, and as a result, they would make fewer costly errors in their work. Hiring more staff to ensure that the same amount of work would be accomplished would not result in additional payroll costs because four-day employees would only be paid 80 percent of the normal rate. In the end, companies would have fewer overworked and error-prone employees for the same money, which would increase company profits. For the country as a whole, one of the primary benefits of offering this option to employees is that it would reduce unemployment rates. If many full-time employees started working fewer hours, some of their workload would have to be shifted to others. Thus, for every four employees who went on an 80 percent week, a new employee could be hired at the 80 percent rate. Finally, the option of a four-day workweek would be better for individual employees. Employees who could afford a lower salary in exchange for more free time could improve the quality of their lives by spending the extra time with their families, pursuing private interests, or enjoying leisure activities. 听力

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写文书 In many organizations, perhaps the best way to approach certain new projects is to assemble a group of people into a team. Having a team of people attack a project offers several advantages.
First of all, a group of people has a wider range of knowledge, expertise, and skills than any single individual is likely to possess. Also, because of the numbers of people involved and the greater resources they possess, a group can work more quickly in response to the task assigned to it and can come up with highly creative solutions to problems and issues. Sometimes these creative solutions come about because a group is more likely to make risky decisions that an individual might not undertake. This is because the group spreads responsibility for a decision to all the members and thus no single individual can be held accountable if the decision turns out to be wrong.
Taking part in a group process can be very rewarding for members of the team. Team members who have a voice in making a decision will no doubt feel better about carrying out the work that is entailed by the decision than they might doing work that is imposed on them by others.
Also, the individual team member has a much better chance to "shine", to get his or her contributions and ideas not only recognized but recognized as highly significant, because a team's overall results can be more farreaching and have greater impact than what might have otherwise been possible for the person to accomplish or contribute working alone. 听力

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写文书 Rembrandt is the most famous of the seventeenth-century Dutch painters. However, there are doubts whether some paintings attributed to Rembrandt were actually painted by him. One such painting is known as attributed to Rembrandt because of its style, and indeed the representation of the woman's face is very much like that of portraits known to be by Rembrandt. But there are problems with the painting that suggest it could not be a work by Rembrandt.
First, there is something inconsistent about the way the woman in the portrait is dressed. She is wearing a white linen cap of a kind that only servants would wear-yet the coat she is wearing has a luxurious fur collar that no servant could afford. Rembrandt, who was known for his attention to the details of his subjects' clothing, would not have been guilty of such an inconsistency.
Second, Rembrandt was a master of painting light and shadow, but in this painting these elements do not fit together. The face appears to be illuminated by light reflected onto it from below. But below the face is the dark fur collar, which would absorb light rather than reflect it. So the face should appear partially in shadow-which is not how it appears. Rembrandt would never have made such an error.
Finally, examination of the back of the painting reveals that it was painted on a panel made of several pieces of wood glued together. Although Rembrandt often painted on wood panels, no painting known to be by Rembrandt uses a panel glued together in this way from several pieces of wood.
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写文书 Endotherms are animals such as modern birds and mammals that keep their body temperatures constant. For instance, humans are endotherms and maintain an internal temperature of 37℃,no matter whether the environment is warm or cold. Because dinosaurs were reptiles, and modern reptiles are not endotherms, it was long assumed that dinosaurs were not endotherms. However, dinosaurs differ in many ways from modern reptiles, and there is now considerable evidence that dinosaurs were, in fact, endotherms.
Polar dinosaurs
One reason for believing that dinosaurs were endotherms is that dinosaur fossils have been discovered in Polar Regions. Only animals that can maintain a temperature well above that of the surrounding environment could be active in such cold climates.
Leg position and movement
There is a connection between endothermy and the position and movement of the legs. The physiology of endothermy allows sustained physical activity, such as running. But running is efficient only if an animal's legs are positioned underneath its body, not at the body's side, as they are for crocodiles and many lizards. The legs of all modern endotherms are underneath the body, and so were the legs of dinosaurs. This strongly suggests that dinosaurs were endotherms.
Haversian canals
There is also a connection between endothermy and bone structure. The bones of endotherms usually include structures called Haversian canals. These canals house nerves and blood vessels that allow the living animal to grow quickly, and rapid body growth is in fact a characteristic of endothermy. The presence of Haversian canals in bone is a strong indicator that the animal is an endotherm, and fossilized bones of dinosaurs are usually dense with Haversian canals. 听力

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写文书 As early as the twelfth century A.D., the settlements of Chaco Canyon in New Mexico in the American Southwest were notable for their "great houses," massive stone buildings that contain hundreds of rooms and often stand three or four stories high. Archaeologists have been trying to determine how the buildings were used. While there is still no universally agreed upon explanation, there are three competing theories.
One theory holds that the Chaco structures were purely residential, with each housing hundreds of people. Supporters of this theory have interpreted Caco great houses as earlier versions of the architecture seen in more recent Southwest societies. In particular, the Chaco houses appear strikingly similar to the large, well-known "apartment buildings" at Taos, new Mexico, in which many people have been living for centuries.
A second theory contends that the Chaco structures were used to store food supplies. One of the main crops of the Chaco people was grain maize, which could be stored for long periods of time without spoiling and could serve as a long-lasting supply of food. The supplies of maize had to be stored somewhere, and the size of the great houses would make them very suitable for the purpose.
A third theory proposes that houses were used as ceremonial centers. Close to one house, called Pueblo Alto, archaeologists identified an enormous mound formed by a pile of old material. Excavations of the mound revealed deposits containing a surprisingly large number of broken pots. This finding has been interpreted as evidence that people gathered at Pueblo Alto for special ceremonies. At the ceremonies, they ate festive meals and then discarded the pots in which the meals had been prepared or served. Such ceremonies have been documented for other Native American cultures. 听力

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写文书 Communal online encyclopedias represent one of the latest resources to be found on the internet. They are in many respects like traditional printed encyclopedias collections of articles on various subjects. What is specific to these online encyclopedias, however, is that any internet user can contribute a new article or make an editorial change in an existing one. As a result, the encyclopedia is authored by the whole community of internet users. The idea might sound attractive, but the communal online encyclopedias have several important problems that make them much less valuable than traditional, printed encyclopedias.
First, contributors to communal online encyclopedia often lack academic credentials, thereby making their contributions partially informed at best and downright inaccurate in many cases. Traditional encyclopedias are written by trained experts who adhere to standards of academic rigor that non-specialists cannot really achieve.
Second, even if the original entry in the online encyclopedia is correct, the communal nature of these online encyclopedias gives unscrupulous users and vandals or hackers the opportunity to fabricate, delete, and corrupt information in the encyclopedia. Once changes have ben made to the original text, an unsuspecting user cannot tell the entry has been tampered with. None of this is possible with a traditional encyclopedia.
Third, the communal encyclopedias focus too frequently, and in too great a depth, on trivial and popular topics, which creates a false impression of what is important and what is not. A child doing research for a school project may discover that a major historical event receives as much attention in an online encyclopedia as, say, a single long-running television program. The traditional encyclopedia provides a considered view of what topics to include or exclude and contains a sense of proportion that onlie "democratic" communal encyclopedias do not. 听力

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写文书 In an effort to encourage ecologically sustainable forestry practices, an international organization started issuing certifications to wood companies that meet high ecological standards by conserving resources and recycling materials. Companies that receive this certification can attract customers by advertising their products as ecocertified. Around the world, many wood companies have adopted new, ecologically friendly practices in order to receive ecocertification. However, it is unlikely that wood companies in the United States will do the same, for several reasons.
First, American consumers are exposed to so much advertising that they would not value or even pay attention to the ecocertification label. Because so many mediocre products are labeled "new" or improved, "American consumers do not place much trust in advertising claims in general.
Second, ecocertified wood will be more expensive than uncertified wood because in order to earn ecocertification, a wood company must pay to have its business examined by a certification agency. This additional cost gets passed on to consumers-American consumers tend to be strongly motivated by price,and therefore they are likely to choose cheaper uncertified wood products. Accordingly, American wood companies will prefer to keep their prices low rather than obtain ecocertification.
Third, although some people claim that it always makes good business sense of American companies to keep up with the developments in the rest of the world, this argument is not convincing. Pursuing certification would make sense for American wood companies only if they marketed most of their products abroad. But that is not the case, American wood businesses sell most of their products in the United States, catering to a very large customer base that is satisfied with merchandise. 听力

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写文书 Toward the end of his life, the Chevalier de Seingalt (1725-1798) wrote a long memoir recounting his life and adventures the Chevalier was a somewhat controversial figure, but since he met many famous people, including kings and writers, his memoir has become a valuable historical source about European society in the eighteenth century. However, some critics have raised doubts about the accuracy of the memoir. They claim that the Chevalier distorted or invented many events in the memoir to make his life seem more exciting and glamorous that it really was.
For example, in his memoir the Chevalier claims that while living in Switzerland, he was very wealthy, and it is known that he spent a great deal of money there on parties and gambling. However, evidence has recently surfaced that the Chevalier borrowed considerable sums of money from a Swiss merchant. Critics thus argue that if the Chevalier had really been very rich, he would not have needed to borrow money.
Critics are also skeptical about the accuracy of the conversations that the Chevalier records in the memoir between himself and the famous writer Voltaire. No one doubts that the Chevalier and Voltaire met and conversed. However, critics complain that the memoir cannot possibly capture these conversations accurately, because it was written many years after the conversations occurred. Critics point out that it is impossible to remember exact phrases from extended conversations held many years earlier.
Critics have also questioned the memoir's account of the Chevalier's escape from a notorious prison in Venice, Italy. He claims to have escaped the Venetian prison by using a piece of metal to make a hole in the ceiling and climbing through the roof. Critics claim that while such a daring escape makes for enjoyable reading, it is more likely that the Chevaliers jailers were bribed to free him. They point out that the Chevalier had a number of politically well-connected friends in Venice who could have offered a bribe. 听力

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写文书 Car manufacturers and governments have been eagerly seeking a replacement for the automobile's main source of power, the internal-combustion engine. By far the most promising alternative source of energy for cars is the hydrogen-based fuel-cell engine, which uses hydrogen to create electricity that, in turn, powers the car. Fuelcell engines have several advantages over internal-combustion engines and will probably soon replace them.
One of the main problems with the internal-combustion engine is that it relies on petroleum, either in the form of gasoline or diesel fuel. Petroleum is a finite resource; someday, we will run out of oil. They hydrogen needed for fuel-cell engines cannot easily be depleted. Hydrogen can be derived from various plentiful sources, including natural gas and even water. The fact that fuel-cell engines utilize easily available, renewable resources makes them particularly attractive.
Second, hydrogen-based fuel cells are attractive because they will solve many of the world's pollution problems. An unavoidable by-product of burning oil is carbon dioxide, and carbon dioxide harms the environment. On the other hand, the only byproduct of fuel-cell engines is water.
Third, fuel-cell engines will soon be economically competitive because people will spend less money to operate a fuel-cell engine than they will to operate an internal-combustion engine. This is true for one simple reason: a fuel-cell automobile is nearly twice as efficient in using its fuel as an automobile powered by an internalcombustion engine is. In other words, the fuel-cell powered car requires only half the fuel energy that the internal-combustion powered car does to go the same distance. 听力

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写文书 The sea otter is a small mammal that lives in waters along the western coast of North America from California to Alaska. When some sea otter populations off the Alaskan coast started rapidly declining a few years ago, it caused much concern because see otters play an important ecological role in the coastal ecosystem. Experts started investigating the cause of the decline and quickly realized that there were two possible explanations: environmental pollution or attacks by predators. Initially, the pollution hypothesis seemed the more likely of the two.
The first reason why pollution seemed the more likely cause was that there were known sources of it along the Alaskan coast, such as oil rigs and other sources of industrial chemical pollution. Water samples from the area revealed increased levels of chemicals that could decrease the otters' resistance to life-threatening infections and thus could indirectly cause their deaths.
Second, other sea mammals such as seals and sea lions along the Alaskan coast were also declining, indicating that whatever had endangered the otters was affecting other sea mammals as well. This is fact again pointed to environmental pollution, since it usually affects the entire ecosystem rather than a single species. Only widely occurring predators, such as the orca (a large predatory whale), could have the same effect, but orcas prefer to hunt much larger prey, such as other whales.
Third, scientists believed that the pollution hypothesis could also explain the uneven pattern of otter decline: at some Alaskan locations the otter populations declined greatly, while at others they remained stable. Some experts explained these observations by suggesting that ocean currents or other environmental factors may have created uneven concentrations of pollutants along the coast. 听力

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写文书 A recent study reveals that people especially young people are reading far less literature, novels, plays, and poems, than they used to. This is troubling because the trend has unfortunate effects for the reading public, for culture in general, and for the future of literature itself.
While there has been a decline in book reading generally, the decline has been especially sharp for literature. This is unfortunate because nothing else provides the intellectual stimulation that literature does. Literature encourages us to exercise our imaginations, empathize the others, and expand our understanding of language. So by reading less literature, the reading public is missing out on important benefits.
Unfortunately, missing out on the benefits of literature is not the only problem. What are people reading instead? Consider the prevalence of self-help books on lists of best sellers. These are usually superficial poorly written, and intellectually undemanding. Additionally, instead of sitting down with a challenging novel, many persons are now more likely to turn on the television, watch a music video, or read a Web page. Clearly, diverting time previously spent in reading literature to trivial forms of entertainment has lowered the level of culture in general.
The trend of reading less literature is all the more regrettable because it is taking place during a period when good literature is being written. There are many talented writers today, but they lack an audience. This fact is bound to lead publishers to invest less in literature and so support fewer serious writers. Thus, the writing as well as the reading of literature is likely to decline because of the poor standards of today's readers. 听力

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写文书 Jane Austen (1775-1817) is one of the most famous of all English novelists, and today her novels are more popular than ever, with several recently adapted as Hollywood movies. But we do not have many records of what she looked like. For a long time, the only accepted image of Austen was an amateur sketch of an adult Austen made by her sister Cassandra. However, recently a professionally painted, full-length portrait of a teenage girl owned by a member of the Austen family has come up for sale. Although the professional painting is not titled Jane Austen, there are good reasons to believe she is the subject.
First, in 1882, several decades after Austen's death, Austen's family gave permission to use the portrait as an illustration in an edition of her letters. Austen's family clearly recognized it as a portrait of the author. So, for over a century now, the Austen family itself has endorsed the claim that the girl in the portrait is Jane Austen.
Second, the face in the portrait clearly resembles the one in Cassandra's sketch, which we know depicts Austen.
Though somewhat amateurish, the sketch communicates definite details about Austen's face. Even though the Cassandra sketch is of an adult Jane Austen, the features are still similar to those of the teenage girl in the painting. The eyebrows, nose, mouth, and overall shape of the face are very much like those in the full-length portrait.
Third, although the painting is unsigned and undated, there is evidence that it was painted when Austen was a teenager. The style links it to Ozias Humphrey, a society portrait painter who was the kind of professional the wealthy Austen family would hire. Humphrey was active in the late 1780s and early 1790s, exactly the period when Jane Austen was the age of the girl in the painting. 听力

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写文书 Private collectors have been selling and buying fossils, the petrified remains of ancient organisms, ever since the eighteenth century. In recent years, however, the sale of fossils, particularly of dinosaurs and other large vertebrates, has grown into a big business. Rare and important fossils are now being sold to private ownership for millions of dollars. This is an unfortunate development for both scientists and the general public.
The public suffers because fossils that would otherwise be donated to museums where everyone can see them are sold to private collectors who do not allow the public to view their collections. Making it harder for the public to see fossils can lead to a decline in public interest in fossils, which would be a pity.
More importantly, scientists are likely to lose access to some of the most important fossils and thereby miss out on potentially crucial discoveries about extinct life forms. Wealthy fossil buyers with a desire to own the rarest and most important fossils can spend virtually limitless amounts of money to acquire them. Scientists and the museums and universities they work for often cannot compete successfully for fossils against millionaire fossil buyers.
Moreover, commercial fossil collectors often destroy valuable scientific evidence associated with the fossils they unearth. Most commercial fossil collectors are untrained or uninterested in carrying out the careful field work and documentation that reveal the most about animal life in the past. For example, scientists have learned about the biology of nest-building dinosaurs called oviraptors by carefully observing the exact position of oviraptor fossils in the ground and the presence of other fossils in the immediate surroundings. Commercial fossil collectors typically pay no attention to how fossils lie in the ground or to the smaller fossils that may surround bigger ones. 听力

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写文书 Every year, forest fires and severe storms cause a great deal of damage to forests in the northwestern United States. One way of dealing with aftermath of these disasters is called salvage logging, which is the practice of removing dead trees from affected areas and using the wood for lumber, plywood, and other wood products. There are several reasons why salvage logging is beneficial both to a damaged forest and to the economy.
First, after a devastating fire, forests are choked with dead trees. If the trees are not removed, they will take years to decompose; in the meantime, no new trees can grow in the cramped spaces. Salvage logging, however, removes the remains of dead trees and makes room for fresh growth immediately, which is likely to help forest areas recover from the disaster.
Also, dead trees do more than just take up space. Decaying wood is highly suitable habitat for insects such as the spruce bark beetle, which in large numbers can damage live, healthy spruce trees. So by removing rotting wood, salvage logging helps minimize the dangers of insect infestation, thus contributing to the health of the forest.
Third and last, salvage logging has economic benefits. Many industries depend upon the forests for their production, and because of this a fire can have a very harmful effect on the economy. Often, however, the trees that have been damaged by natural disasters still can provide much wood that is usable by industries. Furthermore, salvage logging requires more workers than traditional logging operations do, and so it helps create additional jobs for local residents. 听力

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写文书 The cane toad is a large (1.8kg) amphibian species native to Central and South America. It was deliberately introduced to Australia in 1935 with the expectation that it would protect farmers' crops by eating harmful insects. Unfortunately, the toad multiplied rapidly, and a large cane toad population now threatens small native animals that are not pests. Several measures have been proposed to stop the spread of the cane toad in Australia.
One way to prevent the spread of the load would be to build a national fence. A fence that blocks the advance of the toads will prevent them from moving into those parts of Australia that they have not yet colonized. This approach has been used before: a national fence was erected in the early part of the twentieth century to prevent the spread of rabbits, another animal species that was introduced in Australia from abroad and had a harmful impact on its native ecosystems.
Second, the toads could be captured and destroyed by volunteers. Cane toads can easily be caught in simple traps and can even be captured by hand. Young toads and cane toad eggs are even easier to gather and destroy, since they are restricted to the water. If the Australian government were to organize a campaign among Australian citizens to join forces to destroy the toads, the collective effort might stop the toad from spreading.
Third, researchers are developing a disease-causing virus to control the cane toad populations. This virus will be specially designed: although it will be able to infect a number of reptile and amphibian species, it will not harm most of the infected species; it will specially harm only the cane toads. The virus will control the population of cane toads by preventing them from maturing and reproducing. 听力

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写文书 The United Kingdom (sometimes referred to as Britain) has a long and rich history of human settlement. Traces of buildings, tools, and art can be found from periods going back many thousands of years: from the Stone Age, through the Bronze Age, the Iron Age, the time of the Roman colonization, the Middle Ages, up to the beginnings of the industrial age. Yet for most of the twentieth century, the science of archaeology, dedicated to uncovering and studying old cultural artifacts, was faced with serious problems and limitations in Britain.
First, many valuable artifacts were lost to construction projects. The growth of Britain's population, especially from the 1950s on, spurred a lot of new construction in British cities, towns, and villages. While digging foundations for new buildings, the builders often uncovered archaeologically valuable sites. Usually, however, they proceeded with the construction and did not preserve the artifacts. Many archaeologically precious artifacts were therefore destroyed.
Second, many archaeologists felt that the financial support for archaeological research was inadequate. For most of the twentieth century, archaeology was funded mostly through government funds and grants, which allowed archeologists to investigate a handful of the most important sites but which left hundreds of other interesting projects without support. Furthermore, changing government priorities brought about periodic reductions in funding.
Third, it was difficult to have a career in archaeology. Archaeology jobs were to be found at universities or with a few government agencies, but there were never many positions available. Many people who wanted to become archaeologists ended up pursuing other careers and contributing to archaeological research only as unpaid amateurs. 听力

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写文书 In the past century, the steady growth of the human population and the corresponding increase in agriculture and pesticide use have caused much harm to wildlife in the United States, birds in particular. Unfortunately for birds, these trends are likely to continue, with the result that the number of birds in the United States will necessarily decline.
First, as human populations and settlements continue to expand, birds' natural habitats will continue to disappear. Forests, wetlands, and grasslands will give way to ever more homes, mails, and offices. As the traditional areas suitable for birds keep decreasing, so will the size of the bird populations that depend on those vanishing habitats
Second, agricultural activities must increase to keep pace with the growing human population. The growth of agriculture will also result in the further destruction of bird habitats as more and more wilderness areas are converted to agricultural use. As a result, bird populations in rural areas will continue to decline.
Third, as human settlements expand and agriculture increases, the use of chemical pesticides will also increase. Pesticides are poisons designed to kill agricultural and home garden pests, such as insects, but inevitably, pesticides get into the water and into the food chain for birds where they can harm birds. Birds that eat the poisoned insects or drink contaminated water can die as a result, and even if pesticides do not kill birds outright, they can prevent them from reproducing successfully. So pesticides have significantly contributed to declines in bird population, and because there will continue to be a need to control agricultural pests in the future, this decline will continue. 听力

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写文书 In the 1950s, Torreya taxifoha, a type of evergreen tree once very common in the state of Florida, started to die out. No one is sure exactly what caused the decline, but chances are good that if nothing is done, Torreya will soon become extinct. Experts are considering three ways to address the decline of Torreya.
The first option is to reestablish Torreya in the same location in which it thrived for thousands of years. Torreya used to be found in abundance in the northern part of Florida, which has a specific microclimate. Microclimate exists when weather conditions inside a relatively small area differ from the region of which that area is a part. Northern Florida's microclimate is very favorable to Torreya's growth. This microclimate is wetter and cooler than the surrounding region's relatively dry, warm climate. Scientists have been working to plant Torreya seeds in the coolest, dampest areas of the microclimate.
The second option is to move Torreya to an entirely different location, far from its Florida microclimate. Torreya seeds and saplings have been successfully planted and grown in forests further north, where the temperature is significantly cooler. Some scientists believe that Torreya probably thrived in areas much further north in the distant past, so by relocating it now, in a process known as assisted migration, humans would simply be helping Torreya return to an environment that is more suited to its survival.
The third option is to preserve Torreya in research centers. Seeds and saplings can be moved from the wild and preserved in a closely monitored environment where it will be easier for scientists both to protect the species and conduct research on Torreya. This research can then be used to ensure the continued survival of the species. 听力

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写文书 Many consumers ignore commercial advertisements. In response, advertising companies have started using a new tactic, called "buzzing". The advertisers hire people, buzzers, who personally promote [buzz] products to people they know or meet. The key part is that the buzzers do not reveal that they are being paid to promote anything. They behave as though they were just spontaneously praising a product during normal conversation. Buzzing has generated a lot of controversy, and many critics would like to see it banned.
First, the critics complain that consumers should know whether a person praising a product is being paid to praise the product. Knowing this makes a big difference: we expect the truth from people who we believe do not have any motive for misleading us. But with buzzing what you hear is just paid advertising, which may well give a person incorrect information about the buzzed product.
Second, since buzzers pretend they are just private individuals, consumers listen to their endorsements less critically than they should. With advertisements in print or on TV, the consumer is on guard for questionable claims or empty descriptions such as "new and improved." But when consumers do not know they are being lobbied, they may accept claims they would otherwise be suspicious of. This may suit the manufacturers, but it could really harm consumers.
And worst of all is the harmful effect that buzzing is likely to have on social relationships. Once we become aware that people we meet socially may be buzzers with a hidden agenda, we will become less trustful of people in general. So buzzing will result in the spread of mistrust and the expectation of dishonesty. 听力

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写文书 In the United States, it had been common practice since the late 1960s no to suppress natural forest fires. The
"let it burn" policy assumed that forest fire would burn themselves out quickly, without causing much damage. However, in the summer of 1988, forest fires in Yellowstone, the most famous national park in the country, burned for more than two months and spread over a huge area, encompassing more than 800,000 acres. Because of the large scale of the damage, many people called for replacing the "let it burn" policy with a policy of extinguishing forest fires as soon as they appeared. Three kinds of damage caused by the "let it burn" policy were emphasized by critics of the policy.
First, Yellowstone fires caused tremendous damage to the park's trees and other vegetation. When the fires finally died out, nearly one third of Yellowstone's land had been scorched. Trees were charred and blackened from flames and smoke. Smaller plants were entirely incinerated. What had been a national treasure now seemed like a devastated wasteland
Second, the park wildlife was affected as well. Large animals like deer and elk were seen fleeing the fire. Man smaller species were probably unable to escape. There was also concern that the destruction of habitats and the disruption of food chains would make it impossible for the animals that survived the fire to return.
Third, the fires compromised the value of the park as a tourist attraction, which in turn had negative consequences for the ocal economy. .with several thousand acres of the park engulfed in flames, the tourist season was cut short, and a large number of visitors decided to stay away. Of course, local businesses that depended on park visitors suffered as a result. 听力

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写文书 Genetic modification, a process used to change an organism's genes and hence its characteristics, is now being used to improve trees through genetic modification. It is possible to create trees that produce more fruit, grow faster, or withstand adverse conditions. Planting genetically modified trees on a large scale promises to bring a number of benefits.
First, genetically modified trees are designed to be hardier than nature trees; that is, they are more likely to survive than their unmodified counterparts. In Hawaii, for example, a new pest-resistant species of papaya trees has been developed in response to ring spot virus infections that have repeatedly damaged the native papaya tree population. Planting the genetically modified papayas has largely put an end to the ring spot problem.
Moreover, genetically modified trees promise to bring a number of economic benefits to those who grow them. Genetically modified trees tend to grow faster, give greater yields of food, fruit, or other products and be hardier. This allows tree farmer to get faster and greater returns on their farming investment and save on pesticides as well.
Finally, the use of genetically modified trees can prevent overexploitation of wild trees. Because of the growing demand for firewood and building timber, many forests around the world are being cut down faster than they can be replaced. Introducing genetically modified trees, designed for fast growth and high yield in given geographic conditions, would satisfy the demand for wood in many of those areas and save the endangered native trees, which often include unique or rare species. 听力

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写文书 Ethanol fuel, made from plants such as corn and sugar cane, has been advocated by some people as an alternative to gasoline in the United States. However, many critics argue that ethanol is not good replacement for gasoline for several reasons.
First, the increased use of ethanol fuel would not help to solve one of the biggest environmental problems caused by gasoline use: global warming. Like gasoline, ethanol releases carbon dioxide into the atmosphere when it is burned for fuel and carbon dioxide is greenhouse gas: it helps trap heat in the atmosphere. Thus, ethanol offers no environmental advantage over gasoline.
Second, the production of significant amounts of ethanol would dramatically reduce the amount of plants available for uses other fuel. For example, much of the corn now grown in the United States is used to feed farm animals such as cows and chickens. It is estimated that if ethanol were used to satisfy just 10 percent of the fuel needs in the United States, more than 60 percent of the corn currently grown in the United States would have to be used to produce ethanol. If most of the corn were used to produce ethanol, a substantial source of food for animals would disappear.
Third, ethanol fuel will never be able to compete with gasoline on price. Although the prices of ethanol and gasoline for the consumer are currently about the same, this is only because of the help in the form of tax subsidies given to ethanol producers by the United States government. These tax subsidies have cost the United States government over $11 billion in the past 30 years. If the United States government were to stop helping producers in this way, the price of ethanol would increase greatly. 听力

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写文书 Populations of the yellow cedar, a species of tree that is common in northwestern North America, have been steadily declining for more than a century now, since about 1880. Scientists have advanced several hypotheses explain this decline.
One hypothesis is that the yellow cedar decline may be caused by insect parasites, specially the cedar bark beetle. This beetle is known to attack cedar trees: the beetle larvae eat the wood. There have been recorded instances of sustained beetle attacks overwhelming and killing yellow cedars, so this insect is a good candidate for the cause of the tree's decline.
A second hypothesis attributes the decline to brown bears. Bears sometimes claw at the cedars in order to eat the tree bark, which has high sugar content. In fact, the cedar bark can contain as much sugar as the wild berries that are a staple of the bears' diet. Although the bears' clawing is unlikely to destroy trees by itself, their aggressive feeding habits may critically weaken enough trees to be responsible for the decline.
The third hypothesis states that gradual changes of climate may be to blame. Over the last hundred years, the patterns of seasonal as well as day-to-day temperature have changed in northwestern North America. These changes have affected the root systems of the yellow cedar trees: the fine surface roots now start growing in the late winter rather than in the early spring. The change in the timing of root growth may have significant consequences. Growing roots are sensitive and are therefore likely to suffer damage from partial freezing on cold winter nights. This frozen root damage may be capable of undermining the health of the whole tree, eventually killing it. 听力

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写文书 Animal fossils usually provide very little opportunity to study the actual animal tissues, because in fossils the animals' living tissues have been largely replaced by minerals. Thus, scientists were very excited recently when it appeared that a 70-million-year-old fossil of Tyrannosaurus rex (T. rex), a dinosaur, might still contain remains of the actual tissues of the animal. The discovery was made when researchers deliberately broke open the T. rex's leg bone, thereby exposing its insides to reveal materials that seem to be remains of blood vessels, red blood cells, and collagen matrix.
First, the breaking of the fossilized leg bone revealed many small ranching channels inside, which probably correspond to hollows in the bones where blood vessels were once located. The exciting finding was the presence of a soft, flexible organic substance inside the channels. This soft substance may very well represent the remains of the actual blood vessels of T. rex.
Second, microscopic examination of the various parts of the inner bone revealed the presence of spheres that could be the remains of red blood cells, tests showed that the spheres contained iron-a material vital to the role of red blood cells in transporting oxygen to tissues. Moreover, the spheres had dark red centers (substances with iron tend to be reddish in color) and were also about the size of red blood cells.
Third, scientists performed a test on the dinosaur leg bone that showed that it contained collagen. Collagen is a fibrous protein that is a main component of living bone tissue, in which it forms a so-called collagen matrix. Collagen (or its chemical derivatives) is exactly the kind of biochemical material that one would expect to find in association with bone tissue. 听力

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写文书 In 1938 an archaeologist in Iraq acquired a set of clay jars that had been excavated two years earlier by villagers constructing a railroad line. The vessel was about 2,200 years old. Each clay jay contained a copper cylinder surrounding an iron rod. The archaeologist proposed that vessel were ancient electric batteries and even demonstrated that they can produce a small electric current when filled with some liquids. However, it is not likely that the vessels were actually used as electric batteries in ancient times.
First of all, if the vessels were used as batteries, they would probably have been attached to some electricity conductors such as metal wires. But there is no evidence that any metal wires were located near the vessels. All that has been excavated are the vessels themselves.
Second, the copper cylinders inside the jars look exactly like copper cylinders discovered in the ruins of Seleucia, an ancient city located nearby. We know that the copper cylinders from Seleucia were used for holding scrolls of sacred texts, not for generating electricity. Since the cylinders found with the jars have the same shape, it is very likely they were used for holding scrolls as well. That no scrolls were found inside the jars can be explained by the fact that the scrolls simply disintegrated over the centuries.
Finally, what could ancient people have done with the electricity that the vessels were supposed to have generated? They had no devices that replied on electricity. As batteries, the vessels would have been completely useless to them. 听力

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写文书 The zebra mussel, a freshwater shellfish native to Eastern Europe, has long been spreading out from its original habitats and has now reached parts of North America. There are reasons to believe that this invasion cannot be stopped and that it poses a serious threat to freshwater fish populations in all of North America.
First, the history of the zebra mussel's spread suggests that the invasion might be unstoppable. It is a prime example of an invasion made possible by human transportation. From the zebra mussel's original habitats in Eastern Europe, ships helped spread it out along new canals built to connect Europe's waterways. The mussel can attach itself to a ship's bottom or can survive in the water—called "ballast water"-that the ship needs to take on to properly balance its cargo. By the early nineteenth century, the mussel had spread to the whole of Europe. It was later carried to the east coast of North America in the ballast water of ships traveling from Europe. The way ships have spread the zebra mussel in the past strongly suggests that the species will soon colonize all of North America.
Moreover, once zebra mussels are carried to a new habitat, they can dominate it. They are a hardy species that does well under a variety of conditions, and they have a high rate of reproduction. Most important, however, zebra mussels often have no predators in their new habitats, and species without natural predators are likely to dominate their habitats.
Finally, zebra mussels are likely to cause a decline in the overall fish population in habitats where they become dominant. The mussels are plankton eaters, which means that they compete for food with many freshwater fish species. 听力

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写文书 The little ice age was a period of unusually cold temperature in many parts of the world that lasted from about the year 1350 until 1900CE. There were unusually harsh winters, and glaciers grew larger in many areas. Scientists have long wondered what caused the Little Ice Age. Several possible causes have been proposed.
First, the cooling may have been caused by disrupting of ocean currents. Before the Little Ice Age, there was a period of unusually warm weather during which glaciers melted. These melted glaciers sent a large amount of cold freshwater into the Gulf Stream, a large ocean current that strongly affects Earth's climate. Some scientists believe that this freshwater was enough to temporarily disrupt the Gulf Stream. Such a disruption could have caused the Little Ice Age.
Second, volcanic eruption could have caused the Little Ice Age. When volcanoes erupt, they send dark clouds of dust and sulfur gas into the atmosphere. These clouds, which can spread over great areas, block some sunlight form reaching Earth's surface. This can decrease the global temperatures. Scientists know of several volcanic eruption that took place during the Little Ice Age.
Third, substantial decreases in human populations may have contributed indirectly to the cooling of the climate. For a variety of reason (disease, warfare, social disruption), the human population just before the Little Ice Age and during the early part of it was lower than it had been in a long time. Forest trees started growing on fields that were no longer used for agriculture. Since trees absorb carbon dioxide, a greenhouse gas, they decrease the greenhouse effect that keeps Earth warm. With more forest trees carbon dioxide, earth became cooler. 听力

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写文书 When he returned from the expedition, he claimed to have reached the pole on April 7, 1909. This report made him into an international celebrity. Though some historians have expressed doubts that Peary did in fact reach the North Pole, three arguments provide strong support for the truth of Peary's claim.
First, the National Geographic Society put together a committee that was instructed to conduct a thorough investigation of Peary's records and equipment. At the end of the investigation, the committee concluded that Peary's accounts were consistent and persuasive and declared that he had indeed reached the North Pole.
Second, a recent expedition provides support for Peary's claim that he reached the North Pole in only 37 days after setting out from Ellesmere Island off the coast of Greenland. Skeptics used to argue that Peary could not have traveled that fast, since even modern snowmobiles take longer to cover the same distance. However, a British explorer named Tom Avery recently made the same trek in LESS THAN 37 DAYS. IN FACT, Avery used the same kind of dogsled and the same number and breed of dogs as Peary had. Thus, Peary's claims are not impossible, and he very well might have been telling the truth.
Third, there are photographs taken by Peary that support his claim to have reached the North Pole. Measuring the shadows in Peary's photographs makes it possible to calculate the Surf's position in the sNy. The surf's position established from the photographs corresponds exactly to the Surf's position as it should have been at the North Pole on that day. This provides strong evidence that Peary reached the North Pole and took the photographs there. 听力

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写文书 Large numbers of dinosaur fossils have been discovered in deposits on Alaska's North Slope, a region that today experiences an extremely cold, arctic climate. One hundred million years ago, when those dinosaurs were alive, the environment of the North Slope was already inhospitable, especially during the winter when it experienced several months of total darkness. How did the dinosaurs survive the wintertime? Paleontologists have proposed that one of the most common North Slope dinosaurs, the elephant-sized edmontosaur (Edmontosaurus), survived the winter by migrating south to more hospitable regions. Several arguments support the migration hypothesis.
First, the edmontosaur's diet supports the migration hypothesis. Edmontosaurs fed exclusively on plants. Since there would have been no plants growing during the cold and dark North Slope winter, it appears that the edmontosaur must have left for at least part of the year and migrated to more temperate zones to find food.
Second, many edmontosaur skeletons have been unearthed from the same site. This suggests that edmontosaurs lived in herd. Many modern-day migratory animals, such as caribou and buffalo, live and migrate in herds as well. Moving in herds helps animals coordinate their migration. The finding that edmonotsaurs lived in herds further supports the migration hypothesis.
Finally, edmonosaurs were physically capable of migrating long distances. To reach more hospitable regions, the edmontosaur had to migrate about 1,600 kilometers southward. To make such a journey, the edmontosaur needed to move at about five kilometers per hour for several weeks, which certainly could do. These animals could run very fast, reaching speeds up to 45 kilometers per hour. It could have easily used its locomotive power to move to warmer climate during the harsh arctic winter. 听力

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写文书 A little over 2,200 years ago, the Roman navy attacked the Greek port city of Syracuse. According to some ancient historians, the Greeks defended themselves with an ingenious weapon called a "burning mirror", a polished copper surface curved to focus the Sun's rays onto Roman ships, causing them to catch fire. However, we have several reasons to suspect that the story of the burning mirror is just a myth and the Greeks of Syracuse never really built such a device.
First, the ancient Greeks were not technologically advanced enough to make such a device. A mirror that would focus sunlight with sufficient intensity to set ships on fire would have to be several meters wide. Moreover, the mirror would have to have a very precise parabolic curvature (a curvature derived from a geometric shape known as the parabola). The technology for manufacturing a large sheet of copper with such specification did not exist in the ancient world.
Second, the burning mirror would have taken a long time to set the ships on fire. In an experiment conducted to determine whether a burning mirror was feasible, a device concentrating the Sun's rays on a wooden object 30 meters away took ten minutes to set the object on fire; and during that time, the object had to be unmoving. It is unlikely that Roman ships stayed perfectly still for that much time. Such a weapon would therefore have been very impractical and ineffective.
Third, a burning mirror does not seem like an improvement on a weapon that the Greeks already had: flaming arrows shooting at an enemy's ships with flaming arrows was a common way of setting the ships on fire. The burning mirror and flaming arrows would have been effective at about the same distance. So the Greeks had no reason to build a weapon like a burning mirror. 听力

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写文书 A fossil skeleton of a dinosaur called Sinosauropteryx, preserved in volcanic ash, was discovered in Liaoning, China, in 1996. Interestingly, the fossil included a pattern of fine lines surrounding the skeletal bones. Some paleontologists interpret the lines as evidence that Sinosauropteryx was a feathered dinosaur, citing several reasons.
First, the critics point out that the fine lines may not even represent functional structures of a living dinosaur, but rather structures that were formed after the animal's death. After the animal died and was buried in volcanic ash, its skin may have decomposed into fibers. The skin fibers then became preserved as lines in fossils; the lines were misinterpreted as evidence of feathers.
Second, even if the fine lines are remains of real structures of a Snosauropteryx, scientists cannot tell with certainty what part of the dinosaur's anatomy the structures were. Many dinosaurs had frills, ornamental fanshaped structures growing out of some parts of their bodies. Some of the critics argue that the lines surrounding the skeleton are much more likely to be fossilized remains of frills than remains of feathers.
A third objection is based on the fact that the usual functions of feathers are to help animals fly or regulate their internal temperature. However, the structures represented by the lines in the Sinosauropteryx fossil were mostly located along the backbone and the tail of the animal. This would have made the structures quite useless for flight and of very limited use in thermoregulation. This suggests that the lines do not represent feathers. 听力

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写文书 Starting in the 1960s and continuing until the 1980s, sailors in Russian submarines patrolling the North Atlantic and Arctic Ocean would occasionally hear strange sounds. These underwater noises reminded the submarine crews of frog croaks, so they called the sounds "quackers" (from the Russian word for frog sounds). The sources of the sound seemed to be moving with great speed and agility; however, the submarines' sonar (a method of detecting objects underwater) was unable to detect any solid moving objects in the area. There are several theories about what might have caused the odd sounds.
The first theory suggests that the strange noises were actually the calls of male and female orca whales during a courtship ritual. Orca whales are known to inhabit the areas where the submarines were picking up the bizarre noises. Orcas have been studied extensively, and the sounds they make when trying to attract a male are similar to those that the submarines were detecting.
A second idea is that the sounds were caused by giant squid. Giant squids are giant marine invertebrates that live deep in the ocean and prey on large fish. They are difficult to detect by sonar because they have soft bodies with no skeleton. Not much is known about giant squid behavior, but their complex brains suggest they are intelligent animals. It is possible they have the ability to emit sound, and perhaps they approached the submarines out of curiosity.
A third theory suggest the Russian submarines were picking up stray sounds from some military technology, like another country's submarines that were secretly patrolling the area. Perhaps the foreign submarines did not register on sonar because they were using a kind of technology specifically designed to make them undetectable by sonar. The strange froglike sounds may have been emitted by the foreign submarines unintentionally. 听力

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写文书 Carved stone balls are curious type of artifact found at a number of locations in Scotland. They date from the late Neolithic period, around 4,000 years ago. They are round in shape; they were carved from several types of stone; most are about 70 mm in diameter; and many are ornamented to some degree. Archaeologists do not agree about their purpose and meaning, but there are several theories.
One theory is that the carved stone balls were weapons used in hunting or fighting. Some of the stone balls have been found with holes in them, and many have grooves on the surface. It is possible that a cord was strung through the holes or laid in the grooves around the ball. Holding the stone balls at the end of the cord would have allowed a person to swing it around or throw it.
A second theory is that the carved stone balls were used as part of a primitive system of weights and measures. The fact that they are so nearly uniform in size—at 70 mm in diameter—suggests that the balls were interchangeable and represented some standard unit of measure. They could have been used as standard weights to measure quantities of grain or other food, or anything that needed to be measured by weight on a balance or scale for the purpose of trade.
A third theory is that the carved stone balls served a social purpose as opposed to a practical or utilitarian one. This view is supported by the fact that many stone balls have elaborate designs. The elaborate carving suggests that the stones may have marked the important social status of their owners. 听力

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写文书 A huge marine mammal known as Steller's sea cow once lived in the waters around Bering Island off the coast of Siberia. It was described in 1741 by Georg W. Steller, a naturalist who was among the first European to see one. In 1768 the animal became extinct. The reasons for the extinction are not clear. Here are three theories about the main cause of the extinction.
First, the sea cows may have been overhunted by groups of native Siberian people. If this theory is correct, then the sea cow population would have originally been quite large, but hundreds of years off too much hunting by the native people diminished the number of sea cows. Sea cows were a good source of food in a harsh environment, so overhunting by native people could have been the main cause of extinction.
Second, the sea cow population may have become extinct because of ecosystems disturbances that caused a decline in their main source of food, kelp (a type of sea plant). Kelp populations respond negatively to a number of ecological changes. It is possible that ecological changes near Bering island some time before 1768 caused a decrease of the kelp that the sea cows depend on.
Third, the main cause of extinction of the sea cows could have been European fur traders who came to the island after 1741. It is recorded that the fur traders caught the last sea cow in 1768. It thus seems reasonable to believe that hunting by European fur traders, who possessed weapons that allowed them to quickly kill a large number of the animals, was the main cause of the sea cow's extinction. 听力

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