So far no life has been found on Mars, but some experts still think that there ________ some form of life a long time ago．

1. A.
   might be
2. B.
   may have been
3. C.
   should have been
4. D.
   could be

So far no life has been found on Mars, but some experts still think that there ________ some form of life a long time ago.

   A. might be       B. may have been         C. should have been        D. could be