用所给动词的正确形式填空。

1. Did you see the accident                  this morning? (happen)

2. You should get a chair                  on. (sit)

3. There are so many children _________ (play) in the park on Sundays.

4. It's hot. You'd better                   both the windows and the doors. (open)

5. It's not easy                fish with your hands only. (catch)

6. Our teacher asked us                  to page 16. (turn)

7. You must                 to the teacher carefully in class. (listen)

8. He felt _________ (worry) about his son at that time.

9. He found a number of people already _________ (work) in the workplace.

10. Many children trust animals because they know how _________ (keep) a secret.

煤的干馏和煤的气化的定义是什么？