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

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