Hardly had Mike thrown himself into bed when he fell___asleep.A.fast B.quick C.rapid D.sudden 为什么选A?详细点解释谢谢

Hardly had Mike thrown himself into bed when he fell___asleep.A.fast B.quick C.rapid D.sudden 为什么选A?详细点解释谢谢
Hardly had Mike thrown himself into bed when he fell___asleep.
A.fast B.quick C.rapid D.sudden
为什么选A?详细点解释谢谢

Hardly had Mike thrown himself into bed when he fell___asleep.A.fast B.quick C.rapid D.sudden 为什么选A?详细点解释谢谢
fell fast asleep 很快睡着了
这是惯用法,楼主可以专门记忆.

此处用副词修饰，fast有副词词性，其他都是形容词。

fell fast asleep 很快睡着了
fast:迅速的,指动作等的迅速,常用于副词,和rapid通用,但侧重于动的东西.
quick:迅速的,一般用语,意为立即行动,毫不拖延.
rapid:急速的,常指激流,漩涡等
sudden：突然的（显然用在这不合适）

习惯用法。