Every possible means to 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
网友回答
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。