Mr. Smith very naughty when he was a boy. [ ]
A.used to do
B.used to be
C.used to
答案:B
如图、物体受到与水平方向成α角的力F的作用,在水平面上做匀加速直线运动,方向向右,物体受到的摩擦力与拉力的合力方向是
A. 竖直向上
B. 竖直向下
C. 向上偏右
D.向上偏左