Fred is absent from the meeting. I don't know exactly when he is ing back but I'm sure he'll be back ________ Monday.A.onB.byC.untilD.after
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B解析介词by后接表时间的名词，意为“在……之前”如：I have to go back by ten o'clock.我必须在10点钟以前回去。Will you please finish the work by Friday? 请你在星期五之前完成这份工作好吗？