Every possible means to prevent the air pollution but the sky is still not clearA. 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：原句意思是防治污染的每种可能的方式都被使用了，但是天空还是不明亮。首先我们看到means（方法）这个词，它是单数和复数的形式一样的。再看Every possible means，每个可能的方法，是单数。排斥B和D，C表示已经被使用了，现在完成时态与后面的but结果相对应，即先后关系。A被使用了。没有体现先后关系，故选C。