用所给词的适当形式填空，每空不限一词。
1. I think it's _____ (use) to talk with him. It's difficult to change his mind.
2. The terrible snowstorm kept them from _____ (ski) last week.
3. My uncle will come back from London. I'm waiting for his _____ (arrive).
4. _____ (wolf) eat smaller animals when they are hungry.
5. _____ (luck), he was hurt badly.
6. His grandpa was lucky enough to stay _____ (live) during the earthquake.
7. Lushan is so beautiful that he wants to go there for the _____ (two) time.
8. We should encourage people _____ (not cut) trees any more.
9. I _____ ( drive ) to Beijing when my car broke down.
10. The young trees _____ (die) soon if you don't water them in time.
的质点静止于光滑水平面上，从
起，第1秒内受到2N的水平外力作用，第2秒内受到同方向的
的外力作用。下列判断正确的是( )
W
J