Scientists say it may be five years it is possible to test this medicine on human patient

问题补充：

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

答案：

A. since

B. until

(答案→)C. before

D. when

解析：本题考查连词。句意：科学家说可能还要五六年才可能将此药试验在人身上。before引导时间状语从句,表示“过了……就”。