题目：
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.
答案：
1. go 2. to go 3. riding 4. sending 5. (to) do
