Children often play tricks _____ others. [ ]
A. to
B. on
C. at
答案：B
There are ___________ boys. There aren't ___________ books.[ ]
A. enough, too many
B. too many, enough
B．它是近地圆轨道上人造卫星的运行速度
C．它是卫星进入近地轨道的最小发射速度
D．它是人造卫星运动时的最大速度
