1. We were caught in a traffic jam. By the time we arrived at the airport the plane ________.
A. will take off
B. would take off
C. has taken off
D. had taken off
2.Every means ________ but it's not so effective.
A.have been tried
B.has been tried
C.have tried
D.has tried
3.By the time class was over, the rain ________.
A.will stop
B.would stop
C.had stopped
D.have stopped
4. By this summer, Elizabeth ________ here for eight years.
A. will live
B. would have lived
C. will have lived
D. has lived
5. At first, I didn't recognize her because she ________ at least fifty pounds.
A. have lost
B. might lose
C. had lost
D. may lose
答案解析：
1.【解析】考查过去完成时。本句时间标志词为By the time we arrived，等我们赶到的时候，为一般过去时，在过去的某个时间点之前发生的事情，应该用过去完成时。结合句意可知：我们遇到了交通堵塞。我们到达机场时，飞机已经起飞了。故答案选D。
2.【解析】考查现在完成时。从过去开始直到现在，一直在重复、延续或对现在有影响的动作。结合句意可知：各种方法都已经试过了，但都不是很有效。every表示的是每一个，用单三形式。方法被尝试，表被动，用被动语态。故答案选B。
3.【解析】考查过去完成时。本句时间标志词为By the time class was over，等到了下课的时候，为一般过去时，在过去的某个时间点之前发生的事情，应该用过去完成时。结合句意可知：等到了下课的时候，雨就已经停了。故答案选C。
4.【解析】考查将来完成时。将来完成时指到将来的某个时间点完成某事。时间标志词by this summer，表示“到这个夏天”，指将来的某个时间点。结合句意可知：到了这个夏天，伊丽莎白就在这里住八年了。故答案选C。
5.【解析】考查过去完成时。主句谓语动词didn't recognize，为一般过去时，在过去的某个时间点之前发生的事情，应该用过去完成时。结合句意可知：起初，我没认出她来，因为她至少瘦了50磅。故答案选C。