Scientists say it may be one year or so ________ it is possible to test the new medicine on human patients.

A．when 

B．after

C．before 

D．since

Scientists say it may be one year or so ________ it is possible to test the new medicine on human patients.

   A. when              B. after                C. before          D. since

Scientists say it may be one year or so ________ it is possible to test the new medicine on human patients.

1. A.
   when
2. B.
   after
3. C.
   before
4. D.
   since

Scientists say it may be one year or so ________ it is possible to test the new medicine on human patients.