【单选题】Sleep We all know that the normal human daily cycle of activity is of some 7 - 8 hours' sleep alternating with some 16 - 17 hours' wakefulness and that, broadly speaking, the sleep normally coincides ...
A.
because Bonjer's findings are different from Brown's.
B.
because sleep normally coincides with the hours of darkness.
C.
because some people can change their sleeping habits easily.
D.
because shift work in industry requires people to change their sleeping habits.