题目

They prepared man­made rubber as a(n) ________ for the natural rubber that they could no longer obtain. A. preference                           B. assistant C. exchange                             D. substitute 答案：D　句意：他们准备人造橡胶代替他们再也无法获得的天然橡胶。substitute“代用品”，符合句意。preference“偏爱；倾向”； assistant“助手”； exchange“交换；交流”。

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