--______, where is the post office(邮局)? --- Sorry, I’m new here. You can ask that policeman(警察). A. Sorry B. Excuse me C. Hi D. Thank you 答案:B 用40 N的水平力F拉一个静止在光滑水平面上、质量为20 kg的物体,力F作用3 s后撤去,则第5 s末物体的速度和加速度的大小分别是( )A.v=6 m/s,a=0 B.v=10 m/s, C.v=6 m/s, D.v=10 m/s,a=0