You should never base anything on a theory that _________ impractical in practice. 

A. is proved        B. prove            C. proves            D. proved

You should never base anything on a theory that _________ impractical in practice.

A. is proved        B. prove         C. proves       D. proved

You should never base anything on a theory that _________ impractical in practice.

1. A.
   is proved
2. B.
   prove
3. C.
   proves
4. D.
   proved

You should never base anything on a theory that _________ impractical in practice.