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

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：C mean单数复数一个形式。在本题中代表单数。