问题

Scientists say it may be five or six years _____ it is possible to test this medicine to human patients.
A:since
B:after
C:before
D:when
B:after
C:before
D:when
正确答案是:
同类题型
He goes back to our school to visit his teachers every year. —____________.
Five _______ students from different schools took part in the English contest last Saturday.
______is very important for us to learn English well.
I haven’t heard from Henry for a long time.—– What do you think ___________ to him?