1. 用must, mustn't, might, needn't 或could填空。

1. （1） You drop litter into the sea. It is bad.
3. （2） — I use your eraser for a while?

   — Of course you can.

5. （3） Ask Jim. He know about Tony because they are classmates.
7. （4） After a long walk, the old man be tired now.
9. （5） — Must I leave now? It's still raining hard outside.

   — No, you . You can wait till the rain stops.