用所给词的适当形式填空。

1. When              the car             (invent)?

2. What do you think              (be ) the most helpful invention?

3. A stranger was seen              (go) into the teacher's office by the girl.

4. Knocking into players and falling down would be              . (danger)

5. I am              (save) my money so I can buy a new bicycle.

6.             (be) a volunteer is great.

7. I fix up bikes and give               (they)away to children.

8. I love places where the people are really              (friend)

9. I love singers              (whom) write their own music.

10. Teenagers should              (allow) to choose their own clothes.