It is clear that no use can be made ______radium _____we don’t know clearly the characteristics of it.

It is clear that no use can be made ______radium ______we don’t know clearly the characteristics of it.

1. A.
   from; if
2. B.
   of; unless
3. C.
   from; unless
4. D.
   of; if

It is clear that no use can be made ______radium _____ we don’t know clearly the characteristics of it.

A. from; if　 B. of; unless　 C. from; unless　 D. of; if