Every possible means______,but none proves successful.
  A.has been tried    B.tried    C.is being tried    D.has tried

Every possible means______,but none proves successful.
  A.has been tried    B.tried    C.is being tried    D.has tried

Every possible means ____, but none ____ successful．

A. has tried; has proved     B. tried; proves

C. has been tried; proves    D. is being tried; is proved