Question

__we could not have finished the work on time.

A should they not offer to help us B but for their help

想知道A错误在哪里,如何改.答案是B.

Answer 1

从句子的后半段可以推断，本题考查的是对过去虚拟。
A是对将来虚拟的形式。对过去虚拟的从句结构应当是
if ther had not offered to help us
或 had they not offered to help us
所以不选A

供参考。

Answer 2

B but for their help 要不是他们的帮助 ，对过去的虚拟(if 从句使用过去完成时)

should they not offer to help us =if they should not offer to help us 这是对将来的虚拟

Answer 3

没看出A的错， 不过b确实是好选择 干净利落