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.the best
B.at all
C.without
D.a good