用所给词的适当形式填空。

1. She isn't at home. She must ________ (go) to school.

2. It takes us 20 minutes ________ (go) to school by bus.

3. That must be a lot more fun than ________ (ride) the bike.

4. Thank you for ________ (send) me the postcard.

5. I often help my son ________ (do) his homework.

使用砂轮机研磨时，应戴防护眼镜或装设防护玻璃。