Susan _______out _______the door opened and in came three strangers.
A.was just to go; whileB.just went; asC.was just going; as soon asD.was just about to go; when在线课程D
be about to do sth. when是一个固定的句型，表示正要做某事的时候突然发生另外一件事。when此处相当于并列连词，意为“on the point of doing sth.”,“be going to do sth.”等。