如图,质量为m的物体静止在光滑的平台上,系在物体上的绳子跨过光滑的定滑轮,由地面上的人拉着匀速向右以速度v0走动.则绳子与竖直方向夹角由0°变成45°过程中,人做功为( )
--Where does the rain come from? --It comes from the___. A. sun B. clouds C. river
-Excuse me. Is there a shop near here? --Yes, _____. A. there is B. I amC. it is
Tom wants to buy three _____ in the bookstore. A. post cards B. closetC. shoes
--What does she do? --She is ____ actress. A. the B. an C. a
--___ does he go to Beijing? --By train. A. Whhere B. Who C. How