题目

— Would you like Alice or Jane to do the task?     —_______ is up to it. I’m sure they have the ability. A. None            B. Both              C. Neither        D. Either   答案：D 某人用同一滑轮组分别将重1000N和2000N的物体匀速提升5m，动滑轮重200N，绳重和摩擦均不计，则上述两种情况中A.滑轮组的机械效率不相等B.人做的额外功不相等C.人做的总功不相等D.绳子自由端移动的距离不相等．