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

问题描述:

Scientists say it may be five or six years ____ it is possible to test this medicine on human patients.
A.before B.after C.since D.when
这种before 和when还有after老搞不清楚,怎么区别选择?
答案不是B,C额,老选不对这类题目

A
结合整句话的意思:
科学家们说在这种药被尝试用来治病救人之前还需5~6年.