The best way to prevent the air polluting but the sky is still not clear.A. is usedB. a

问题补充：

The best way to prevent the air polluting , but the sky is still not clear.

A. is used

B. are used

C. has been used

D. have been used

答案：

A. is used

B. are used

(答案→)C. has been used

D. have been used

解析：考查主谓一致与语态。根据上下文可知最好的方法已经被使用了，但是天空仍然不晴朗。可以反应出过去发生的事情对现在的影响。the best way是单数形式，所以谓语动词用单数。句意：阻止空气污染的最好的方法已经被使用了，但是天空仍然不晴朗。