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

问题补充：

I was about to go to bed someone knocked at the door.

A. when

B. then

C. at the time

D. and

答案：

(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。