He came back late, which time all the guesses had already left.A. byB. atC. duringD. aft

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(答案→)A. by
B. at
C. during
D. after
解析：介词辨析.在表示时间时,by在……之前；at表示在具体时刻；during表示在……期间；after表示在……之后。句意：他回来晚了，在那之前客人们已经离开了。选A。