Scientists say it will be five or six years it is possible to test this medicine on huma

问题补充：

Scientists say it will be five or six years it is possible to test this medicine on humans.

A. after

B. before

C. since

D. when

答案：

A. after

(答案→)B. before

C. since

D. when

解析：

试题分析:句意：科学家说还需要六年的时间才有可能在人身上实验这种药物。A在……之后；B在……之前；C自从；D当……时候。固定句型：it will be +时间段+before从句“还要多久才”。答案选B。