题目内容
【题目】Every possible means __________ to prevent air pollution, but the sky is still not clear.
A. are being used
B. have been used
C. has been used
D. had been used
【答案】 C
【解析】本题考查时态、语态和means的单复数用法。means有every修饰, 一定是单数意义,排除A、B两项。D项时态错误。由句子的意义可知答案为C。
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