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they must have finished the work by the end of last month,___?a,mustn't they b,haven't they c,hadn't they d,didn't they

题目详情
they must have finished the work by the end of last month,___?
a,mustn't they b,haven't they c,hadn't they d,didn't they
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答案和解析
选C,当情态动词must表推测时,其反义疑问句要根据实际情况来判断,本题的时间状语是by the end of last month,应与过去完成时连用,还原一下本题应为:They had finished the work by the end of last month.所以要选C.