A:(﹣∞, 14 )
B:(0, 12 )
C:(0, 14 )
D:( 12 ,+∞)
填上合适的分数。____,____,____。
It's generous __ ___ them to donate money to UNICEF. They think it important _____ them to help poor children go to school.
下列简单机械中,定滑轮、扳手、筷子属于省力杠杆的是____ ;一台起重机将重3600N的货物提高5m的过程中,货物的重力势能____ (填“增加”、“减少”或“不变”),如果总功为30000J,则起重机的机械效率为 ____ .