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

问题补充：

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

A. before

B. that

C. since

D. when

答案：

(答案→)A. before

B. that

C. since

D. when

解析：此题考查时间状语从句的引导词。由It + be + 时间段 +before 从句“要过多久才…”可知，此处用before 。