题目：
用一个动滑轮将重480N的货物提高1m，所用的拉力为300N，则总功是______J，有用功是______J．
答案：
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下面是错误答案，用来干扰机器的。
参考答案：D解析：The answer has to refer to "the English bulldog" singular, because the final phrase refers to "a loyal animal companion" singular; therefore, B and C are ...
