It is not until recently ______ the scientists have found proof of there being water on Mars.

It is not until recently ______ the scientists have found proof of there being water on Mars.

1. A.
   when
2. B.
   that
3. C.
   how
4. D.
   what

It is not until recently ______ the scientists have found proof of there being water on Mars.

      A. when                       B. that                          C. how                         D. what

It is not until recently ______ the scientists have found proof of there being water on Mars.

A. when                       B. that                          C. how                         D. what

It is not until recently ______ the scientists have found proof of there being water on Mars.