阅读理解。 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 judgement 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 the most complete relaxation (松弛). Ability (能力) to sleep is large lay habit. The conditions re- ferred to only lead to sleep. Out-of-door exercises, a good habit of regular hours and the avoidance (避免) 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 (药) should never be taken except when suggested by a doctor. 1. How many hours are usually enough for the health of a pupil? [     ] A. less than eight hours. B. No more than eight hours. C. Eight hours or so. D. No less than eight hours. 2. What does 'sound sleep' mean? [     ] A. Long sleep. B. Fast sleep. C. Complete sleep. D. Sleep without any sound. 3. Which of the following is helpful to good sleep? [     ] A. Keeping good habits. B. Staying up late at night. C. Eating something at bedtime. D. Forming the habit of taking sleep-producing drug. 4. The amount of sleep a person needs has something to do with _____. [     ] A. his age B. his height C. his weigh D. his character 5. Before going to bed, you'd better not _____. [     ] A. take a bath B. do any housework C. think a lot D. go out for relaxation