题目

 I’m new here. Could you tell me         , please?    A. when does the first bus arrive        B. when the first bus arrives C. when did the first bus arrive           D. when will the first bus arrive 答案：B如图所示装置，物重30N，在水平向右的拉力F=3N的作用下，以0.1m/s的速度匀速前进，不计绳重，滑轮重及绳与滑轮间的摩擦，则重物G与水平地面的摩擦力是________N，拉力F的功率是________W．