Every possible means to prevent the air pollution but the sky is still not clear. A. is

问题补充：

Every possible means to prevent the air pollution, 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： 考察主谓一致。本题主语中的名词means是一个单复数同形的名词，要在具体语境中判断是单数还是复数。本句中的形容词every后面通常要节单数名词，故使用AC，根据句义说明这是已经发生的事情，故使用现在完成时。句义：每一种可能的方法都已经被使用来预防空气污染，但是天空仍然不晴朗。故C正确。