Identify the sentences in which the relative pronoun can be omitted.


a) Liposuction is a process by which fat is removed from a person's body.

b) Lena is the person who I trust to take care of my children.

c) That is the teacher who helped me before the test.


d) I don't know the reason why you treated me like that.

e) I told her about the game which I bought for the children.

f) They came here to get the activity which we prepared.