I ________ go to bed when I heard someone knocking at the door．
A.was about toB.had just toC.wouldD.would like to在线课程A
解析：
be about to…when…是一个常用句型，意为“正准备……这时……”。when作“这时”讲时，起并列连词的作用，连接两个并列分句。