Every possible meansto prevent the air pollution but the sky is not still clear.A. is use

问题补充：

Every possible meansto prevent the air pollution, but the sky is not still 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：考察主谓一致。Means方法，方式；是一个单复数同形的名词，在本句中前面有every修饰，说明means是一个单数名词，故使用单数形式，根据句意可知是已经被使用来预防空气污染，故C正确。