案例分析题以下短文中共包含5个未完成的句子，针对每个句子中的空缺部分有5个选项，请从A、B、C、D、E五个选项中选出正确选项。
Enough sleep is important to health.The amount of sleep needed depends on the age of the person and the conditions in which sleep takes 21（）.The young may need more sleep than the old，but generally eight hours are enough for the health of grown-ups.Some can do with less than this amount but others may need more.Everyone knows his own 22（）.It is then a matter of how to satisfy it.Sleep should always be enough to make one relaxed and ready for a day’s work.
Fresh air is necessary to sound sleep.It is not without reason for some people to 23（）that it is practical to sleep in the open air.When a person can keep himself warm，out-of-door sleeping probably gives the body a 24（）complete relaxation.
Ability to sleep is largely a habit.Out-of-door walking，a good habit of regular drinking and the avoidance of late eating are all helpful to sound sleep.Such factors are largely within the 25（）of people themselves. 25应选（）