1.an

2.an

3.a

4.zero

5.an

6.a

7.a

8.a

1.Intelligent relatives have read all the books i have.

2.cunning students have introduced themselves as another boys.

3.Generous people ordera lot of dishes in the cafe.

4.cold cellars were closed for summer

5.тут никак нельзя. Ну как бы в задании указано что сделать во множ.числе там где возможно