I did all the exercises through Part1 to Part5.

问题描述:

I did all the exercises through Part1 to Part5.
Oh,you really___;only Part 1 is required.
A.didn't need to.B.needn't have
为什么A不能选.A.B有什么区别

didn't need to do sth是陈述某事不需要做,不一定做了,needn't have done sth是某事不需要做却已经做了,这道题中明显属于第二种情况,所以应该选B.