A writer has to ____ his imagination and experience to produce good works. A. draw upon B. pick up C. break down D. make out 答案:A用一个定滑轮和一个动滑轮组成的滑轮组把重150N的物体匀速提升1m,不计摩擦和绳重,滑轮组的机械效率为60%.则下列选项正确的是
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A.拉力大小一定是125N B.有用功一定是150J
C.总功一定是250J D.动滑轮重一定是100N