중2문법 관련문제중에

다음중 that 의 쓰임이 어법상 어색한것은?
1.That he failed in the exam was true
2.I know that she is unmarried
3.The rumer is that he loves his teacher.
4.This is a good book expect that it's expensive
5.I really want to know that he will succeed or not
답이 5 번이라는데 이해가 안되네요 ㅠ 빠른답변부탁해요