Enough sleep is important to health.The amount of sleep（21）depends on the age of the per-son and the conditions in which sleep（22）.The young may need more sleep than the old,hut（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 relaxed(松弛)and ready for（26）work.
Fresh air is（27）to sound sleep(酣睡).It is not（28）reason for some people to insist 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 refen-ed 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（33）of any person.A bath at（34）,neither?hot nor cold but of body temperature,may be helpful to sleep.Sleeping pills should never be taken?except when suggested by（35）.
第（23）题选
A.seldom
B.sometimes
C.always
D.generally