I was about to go to bed someone knocked at the door.A. whenB. thenC. at the timeD. and

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(答案→)A. when
B. then
C. at the time
D. and
解析：考查固定搭配。be about to do sth. when sb. did sth.意为“正要做某事时，这时，有人......”。句意“我正要去睡觉时，这时有人敲门。”故选A。