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_____.