Ididn'tmanagetoworkouttheproblem

发布时间：2020-07-27 12:19:12

I didn't manage to work out the problem ________ the teacher had explained now.A.untilB.unlessC.whenD.before

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A解析从原句时态及句意可知，用until表时间。unless表示条件，相当于if not。
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