2. I had worn blue shoes.
3. The table had been cleaned by Joe.
4. The key had been lost by us.
5. A fight had been started by them.
7. The window had not been closed by me.
8. The paper had not been bought by them.
9. I had not been noticed by her.
10. Had the problem been solved by her?
The answer is c. A pronoun takes place of a noun and so they is also plural.
