That의 용법, 어떻게 구분하나요?

다음 문장에서 that의 쓰임이 다른 것은 무엇인가요?
1) It was the water that he lost here.
2) It was on Friday that she visited me.
3) It was the best book that I've ever read.
4) It was Susan that I met at the airport.
5) It was Hillary that climbed Mt.Everest first.
나머지는 It...that 강조 구문 같고 3번은 관계대명사 같은데, 어떻게 구분하면 좋을까요? ㅠ