Question

Every possible means ________ to prevent water pollution, but we still have not enough clean water．

[　　]

A．

has been used

B．

are used

C．

were used

D．

have been used

Answer 1

答案：A
解析：

考查时态和主谓一致。means前有every修饰，谓语动词只能用单数，且根据语境we still have not enough clean water可知到现在为止我们已经使用了每一种可能的方法，故选A。