通读下面的短文，掌握其大意。然后，从每小题的四个选项中选出最佳选项。

　　Enough sleep is important to health. The amount of sleep needed　21　on the age of the person and the conditions in which sleep　22　. The young may need more sleep than the old, but　23　eight hours are enough for the health of grown-ups. Some can do with less than this amount, but　24　may need more. Every person knows his own need. It is then a matter of how to　25　it. Sleep should be always enough to make one bring back his strong and get ready for　26　work.

　　Fresh air is　27　to sound sleep. So it is not　28　reason for some people to think that it is practicable to sleep in the open air. 　29　a person can keep himself warm, out-of-door sleeping probably gives the body　30　complete relaxation.

　　Ability to sleep is largely a habit. Out-of-door　31　, a good habit of regular drinking and the avoidance of late eating and　32　are all helpful to sound sleep. Such factors are largely within the　33　of any person. A bath at　34　, neither hot nor cool but of body temperature, may be helpful to sleep. Sleep producing drug（帮助睡眠的药物）should never be taken except when suggested by　35　.

____.

• A.falls down
• B.breaks out
• C.wakes up
• D.takes place