virtuous circle/cycle vs. vicious circle/cycle
辨析：这就是大家所常说的“良性循环”和“恶性循环”了。留学文书里使用后者的概率较大，一般描绘一个不利的客观环境或一系列的失利，为后面的逆袭做好铺垫。
例句：
Pushing companies to innovate is a virtuous circle.
At home a vicious circle starts as finances are drained, increasing the poverty of the household and driving members into a lifestyle that carries a high risk of becoming infected with HIV.