题目

There are many tall green trees on       side of the street. A. either,         B. both        Call             D. every 答案：A解析: 略如图所示，用滑轮组把重为200N的货物在10s内匀速提高到9m高的楼上，所用拉力为80N，则总功和滑轮组的机械效率分别是（　　）A．1800J  83.3%B．2160J    80%C．2160J    83.3%D．1800J     80%