1.Scientists say that it may be five or six years ______ it is possible to test this
medicine on human patients.
A. since B. afte C. before D. when
2. _____, the experiment will be successful.
A. If carefully doing B. If it doing carefully
C. If carefully done D. If doing carefully
3.Was it in the white building, if I may ask, _____ he made a speech to hundreds
of people?
A. that B. where C. which D. when
4.---She should not have done that sort of thing.
---Whatever she did was reasonable, _____ to what you have done. Besides, it's
none of your business. Get down to _____ your lessons.
A. compared; doing B. comparing; doing
C. compared; do D. comparing; do
哪位老师给我讲讲为什么选择c,c,a,a啊

1.翻译过来你就明白为什么是before了：（我尽量直译）科学家说，在这个药物应用到临床试验之前，还得等上五六年
2.if引导的虚拟语气，需要用完成时态
3.that引导的定语从句，修饰the white building，因为前面已经有in，所以不用where
4.前者是时态一致，用过去时，后者是to后面跟进行时态，表示命令语气