Ответ:

2 - A doctor is someone whom/who you go to when you are ill.

3 - Revision is something that/which you do before the exam.

4 - Junkfood is something that/which people eat when they don't want to cook.

5 - Leather is something that/which we use to make boots.

6 - An aspirin is something that/which you take when you've got a headache.

7 - An expert is someone whom/who you go to when you have a difficult question.

8 - An axe is something that/which people use to cut trees.