如果用which 必须得是定语从句which is to be done by them. which to be done不是个句子,
to be done 在句子里做定语,修饰前面的work
B应该改成which will be done
A是用不定式来解释这个“work”
如果用B,就是定语,定语中必须有主谓宾,B没有谓语,因此不对
WHICH后面必须加一个句子,然而选项只有to be done 所以不能加WHICH,如果用WHICH 要改成WHICH IS TO DE DONE BY THEM。