4.He was just falling asleep------he heard someone knocking at the door .A .while B.when C .before D.after

4.He was just falling asleep------he heard someone knocking at the door .A .while B.when C .before D.after
4.He was just falling asleep------he heard someone knocking at the door .
A .while B.when C .before D.after

4.He was just falling asleep------he heard someone knocking at the door .A .while B.when C .before D.after
B
首先,根据句意,C和D不应该选.
然后比较A和B,A后面主要是持续性动词,B后既可以用持续性又用非持续性动词.而when还有一个特殊用法,还表示突然发生某事,意为“正在……忽然”；也可以表示过早发生某事,意为“还没（刚刚）……就”表示出现意想不到的情况.这里就是这种用法.
另外还要注意when的特殊用法和特殊词义.