A.He had different teachers every term. B.He could learn different things from different teachers. C.He enjoyed his course very much although it lasted five years. D.He had to do a lot of drawing at his college.
【单选题】如图所示 , 用 20 N 的力 F 拉着重为 100 N 的物体 , 使其沿水平面以 2 m/s 的速度匀速运动了 5 s, 若每个滑轮重 10 N, 忽略绳重和滑轮轴的摩擦 , 则下列说法正确的是 ( ) A. 拉力 F 做的功为 200 J B. 拉力 F 的功率为 120 W C. 滑轮组对物体做的有用功为 400 J D. 滑轮组对物体所做有用功的功率为 60 W