Question

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

1. A.
   has been used
2. B.
   are used
3. C.
   were used
4. D.
   have been used

Answer 1

A
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。