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 place. The young may need more sleep than the old, but usually eight hours are enough for the health of grown-ups. Some can do with less than this amount, but others may need more. Every person knows his own need. It is then a matter of good judgment  to satisfy his need. Sleep should always be enough to make one bring back his strength and get ready for a day's work. Fresh air is necessary to sound sleep. It is not without reason for some people to think that it is practicable to sleep in the open air. When one can keep himself warm, out-of-door sleeping probably gives the body its most complete relaxation . Ability  to sleep is largely a habit. The conditions referred to only lead to sleep. Out-of-door exercises, a good habit of regular hours and the avoidance  of late eating and worry, which are largely within the control of any person, are all helpful to sound sleep. A bath at bedtime, neither hot nor cool but of body temperature, may be helpful to sleep. Sleep-producing drug  should never be taken except when suggested by a doctor. The amount of sleep a person probably needs has something to do with  _  .
his character his control his age his habit
Question: 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 place. The young may need more sleep than the old, but usually eight hours are enough for the health of grown-ups. Some can do with less than this amount, but others may need more. Every person knows his own need. It is then a matter of good judgment  to satisfy his need. Sleep should always be enough to make one bring back his strength and get ready for a day's work. Fresh air is necessary to sound sleep. It is not without reason for some people to think that it is practicable to sleep in the open air. When one can keep himself warm, out-of-door sleeping probably gives the body its most complete relaxation . Ability  to sleep is largely a habit. The conditions referred to only lead to sleep. Out-of-door exercises, a good habit of regular hours and the avoidance  of late eating and worry, which are largely within the control of any person, are all helpful to sound sleep. A bath at bedtime, neither hot nor cool but of body temperature, may be helpful to sleep. Sleep-producing drug  should never be taken except when suggested by a doctor. The amount of sleep a person probably needs has something to do with  _  .
A. his character
B. his control
C. his age
D. his habit
Answer: