根据句意和首字母或汉语提示，将单词的正确形式写在题中相应的横线上。

1．We can enjoy f         air in the countryside.

2．Rush hour is the time of day when t         is very heavy.

3．Listening to music is a way of           （放松）yourself.

4．It’s hard to          （想象）how they can improve English so quickly.

5．The astronauts           （描述）what they had seen in space.