어법상 어색한 것 찾아주세요.

1. 다음 중 어법상 어색한 문장은?
① it is difficult for her to pass exam.
② it was nice of you to help the lost child.
③ it was stupid of him not to believe my words.
④ it was good for you to obey your parents.
⑤ it was careless of you to forget your homework.

1번 exam 앞에 관사가 없는게 걸리구요
4번은 was를 쓴게 걸리네요.

뭐가 답일까요?@_@