____________ he comes or not makes no difference. (Whether; If)
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We went to the museum, ____________ was very crowded. (which; when)
He'll answer her letter ____________ he receives it. (as soon as; as if)
The day will come ____________ peace-loving people enjoy a world without war. (when; but)
两种不同的商品能够按照一定比例进行交换的原因是：
A. 他们都有价值
B. 它们都有效用
C. 它们都有交换价值
D. 它们都是劳动产品
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