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

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

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

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

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

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

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

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

Scientists say it may be five 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．

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

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

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

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

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