用单词的适当形式填空。

1. My aunt _____ (teach) me to play the violin when I was a child.

2. Look! The people in the river ______ (swim).

3. Tree-planting Day is on the ______ (twelve) of March every year.

4. Stephen spends about half an hour ______ (swim) every Saturday afternoon.

5. I don't like travelling by train. I think it's ______ (safe) to travel by train.

6. —I'm tired. 

    —So am I. We'd better stop ______ (walk) and have a rest.

7.  Plants need CO₂ when they grow, so they can make CO₂ ______ (little) in the air.