Relative pronouns: "who" and "which/that" (object).Look at the sentences. Which relative

Relative pronouns: "who" and "which/that" (object).
Look at the sentences. Which relative pronouns can you leave out? Cross them out.
1) He's the boy who I met last week.
2) He's got a bike which is better than mine.
3) I liked the book which I borrowed from the library.
4) Those are the people who live by the sea.
5) That's the man who mended our car.
6) We went to a place which we had been to before.
Буду благодарна за помощь! Прошу отвечать юзеров, превосходно разбирающихся в англ. языке.