英语it或which的问题

题目

英语it或which的问题
1.Tom's mother kept telling him that he shoud work harder,but ___ didn't help.
A.he B.which C.she D.it
2.The Parkers bought a new house but ___ will need a lot of work before they can move in.
A.they B.it C.one D.which
请问是不是都选it(which),那为什么不选which(it),请问什么时候又要选which(it)?

答案

1 D
2 B
which只有在定语从句里才用到,上面两题都有连词but,前后句子是对等的,所以应该选it作主语用.