Scientists say it may be five or six years ________ it is possible to test this medicine on human patients．

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

Scientists say it may be five or six years _______ it is possible to test this medicine on human patients.

A. since B. after?

C. before D. when?

Scientists say it may be five or six years ____ it is possible to test this medicine on human patients.

       A．since B．a(chǎn)fter  C．before      D．when      

Scientists say it may be five or six years __________it is possible to test this medicine on human patients.

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

Scientists say it may be five or six years_______it is possible to test this medicine on human patients.

Scientists say it may be five or six years ____ it is possible to test this medicine on human patients.

Scientists say it may be five or six years _____ it is possible to test this medicine to human patients.

Scientists say it may be five or six years _____ it is possible to test this medicine on human patients.

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