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

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