I was about to go to bed one of my old classmates made a call to me. A. whenB. asC. while

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(答案→)A. when
B. as
C. while
D. that
解析：考查固定句式；固定句式be about to do sth when…正要做某事在这时…；句子的when是一个并列连词，表示“在那时…”；句意：我正要睡觉，在这时我的一个老同学来看我。故A正确。