单项选择题

M: How come David is always so full of energy
W: He has a strange but highly effective way of sleeping.
M: What is that
W: He takes a short sleep for an hour every six hours and has a total of four hours of sleep each day.
M: Where did he get that strange idea
W: He read flora a book which said it was the best way for human beings, and he believed it.
M: How many hours do you sleep a day
W: I need at least seven hours. I once tried to follow David’s example, but it never worked out for me.
M: If I sleep during the day, I can never wake up.
W: Not everyone is a David I guess.

How many hours does David sleep a day().

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