【单选题】

Enough sleep is important to health. The amount of sleep (56) depends on the age of the person and the conditions in which sleep (57) . The young may need more sleep than the old, but (58) eight hours are enough for the (59) of grown-ups. Some can (60) with less than this amount but (61) may need more.Every person knows his own (62) It is then a matter of how to (63) it. Sleep should always be (64) to make one relaxed and ready for (65) work.

Fresh air is (66) to sound sleep. It is not (67) reason for some people to (68) that it is practical to sleep in the open air. (69) a person can keep himself warm, (70) sleeping probably gives the body (71) complete relaxation.
Ability to sleep is largely a (72) . Out-of-door (73) , a good habit of regular (74) , the avoidance(避免)of late eating, and (75) are all helpful to sound sleep.
A．deep
B．enough
C．short
D．long