请问下面这几道题为何选了横线上的答案？为什么？

请问下面这几道题为何选了横线上的答案？为什么？
1. I'd rather you_D__for him.
答案是A,do it B,to do it. C,doing it. D.did it .
2.we all felt that___D__the highest praise.
A.were B.is C,be D,to be.
3.She worries for fear that the child___B__hurt.
A.would be B.be C,was D.were
4.There ___D___nothing else we could do,we left.
A.was B.be C,had D.Being.
还有下面这句话错在哪里？
that coat is too heavy in such a mild day.