The old man needs at most five hour's sleep a night,but he -

The old man needs at most five hour's sleep a night,but he ＿ for over seven hours tonight.
A.has fallen asleep B.has slept C.has gone to bed D.has gone to sleep
请详细说明选哪个,以及相应的语法知识.
二楼的麻烦注意点，不是来给你发这种东西的。