题目：
滑轮组由两个动滑轮和两个定滑轮组成，一个工人站在地上用600N的力通过滑轮组匀速提升2000N的货物，绳子自由端4s内拉下4m，不计绳重和摩擦，此时滑轮组的机械效率为_____；若用此滑轮组提升3200N的货物时的机械效率又为_____．
答案：
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BCD
滑轮组由两个动滑轮和两个定滑轮组成，一个工人站在地上用600N的力通过滑轮组匀速
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