1.By 1980, they will have to produce at least 20 milion barrels of oil
a day if the West ______.
A.is to avoid a crippling energy crisis
B.avoids a crippling energy crisis
ANS:A
想请问为什么不能用B呢？两者的差别在哪里呢？
2.I could not finish the homework ____.
A.by he had come
B.until he has come
c.when he comes
D.before he came
ANS:D
这题想请问为什么不能使用A～
谢谢板上英文高手的解答！！
关于时式的用法好混乱还请大家不吝赐教！