A.定滑轮省力,动滑轮改变方向 B.定滑轮改变方向,动滑轮省力 C.定滑轮、动滑轮都能改变方向 D.定滑轮、动滑轮都能省力
A.70% B.75% C.85% D.95%
A.1/4 B.1/3 C.1/2 D.3/4