He was just falling asleep there was a loud knock on the door.A. whileB. whenC. thenD. un

问题补充：

He was just falling asleep there was a loud knock on the door.

A. while

B. when

C. then

D. until

答案：

A. while

(答案→)B. when

C. then

D. until

解析：句意：在他刚刚睡着的时候，有人在大声地敲门。be doing sth …when …正在做某事的时候，突然…，固定句型，此处的when表示“突然”，故选B