根据句子的意思和首字母提示，用适当的词填空，使句子的意思连贯与正确。

1. Your trousers are d_____. You must wash them.

2. Have you seen my pen? I can't f_____ it.

3. We usually have three m_____ a day.

4. S_____ is the best season for planting trees.

5. We listen with our e_____.

6. Fifteen and twenty-five is f_____.

7. It often s_____ in the north of China in winter.

8. —Have you ever been to Xi'an?

     —Yes, I have been there t_____.