根据句意，用括号内所给动词的适当形式填空。
1. Every year foreigners _______(come) to China to learn Chinese.
2. —why didn't you come and open the door for me, dear?
—Oh, sorry, I _______(cook) in the chicken. I didn't hear you.
3. Some food _______(taste) good, but it is usually bad for our health.
4. Look! It's raining heavily. Why mot _______(take) a raincoat with you?
5. Dear Erie, your room is too dirty, It should _______(clean) right away.
6. The film _______(begin) by the time I got to the cinema.
7. —Keep quiet, kids, Dad _______(sleep) in the next room.
—OK, Mum.
8. As teenagers, we're old enough _______(help) with housework We can help set the table,
wash the dishes and clean our own rooms.
9. We know each other well because we _______(be) friends since tea years ago,
10. David preferred _______(sell) newspapers to doing housework when he was young.
（m），它的速度随时间t变化的关系为
（m/s），该质点在t=0到t=2s间的平均速度和t＝2s到t=3s间的平均速度的大小分别为（ ）