Scientists say it may be five or six years it is possible to test this medicine on human patients.
A. when
B. after
C. before
D. since
网友回答
A. when
B. after
(答案→)C. before
D. since
解析：考查固定句型：It will/may be+一段时间+before….过多长时间才/就….句意是：科学家说可能要5到6年这种药才能在病人身上做实验。选C。