通读下面的短文，掌握其大意。然后，从每小题的四个选择项中选出可填入相应空白处的最佳选项。

　　Enough sleep is important to health. The amount of sleep 　21　 depends on the age of the person and the conditions in which sleep 　22　. The young may need more sleep than the old, but generally eight hours are enough for the health of grown-ups. Some can 　23　 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 always be enough to make one relaxed（松弛）and ready for 　26　 work.

　　Fresh air is 　27　 to sound sleep（酣睡）. It is not without reason for some people to 　28　 that it is practical 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. The conditions referred to only lead to sleep. 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 control of any person. A bath at 　33　, neither hot nor cold but of 　34　 temperature, may be helpful to sleep. Sleeping pills should never be taken except when suggested by 　35　.

____.

• A.any time
• B.bed time
• C.midnight
• D.lunch time