Ammonia______as the refrigerant in the first practical refrigerator, made by Kar

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问题 Ammonia______as the refrigerant in the first practical refrigerator, made by Karl von Linde in 1876.

选项 A、used
B、had used
C、is used
D、was used

答案 D

解析 The sentence needs a passive-voice verb; was used agrees in tense with the other verb in the sentence, made, and the time marker 1876.

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