A、Because their bodies are fatigued B、Because some people have a greater inertia than others. C、Because they fall asleep quickly

There must be few questions on which responsible opinion is so utterly divided as on that of how much sleep we ought to have. There are some who think we can leave the body to regulate these matters for itself. "The answer is easy," says Dr. A. Burton. "With the right amount of sleep you should wake up fresh and alert five minutes before the alarm rings." If he is right many people must be undersleeping, including myself. But we must remember that some people have a greater inertia than others. This is not meant rudely. They switch on slowly, and they are reluctant to switch off. They are alert at bedtime and sleepy when it is time to get up, and this may have nothing to do with how fatigued their bodies are, or how much sleep they must take to lose their fatigue.
    Other people feel sure that the present trend is towards too little sleep. To quote one medical opinion, "Thousands of people drift through life suffering from the effects of too little sleep"; the reason is not that they can’t sleep. This in itself, of course, need not be a bad thing, what could be disastrous, however, is that we should press too quickly towards this goal, sacrificing sleep only to gain more time in which to jeopardize our civilization by actions and decisions made weak by fatigue.
    Then, to complete the picture, there are those who believe that most people are persuaded to sleep too much. One can see the point of this also. It would be a pity to retard our development by holding back those people who are gifted enough to work and play well with less than the average amount of sleep, if indeed it does them no harm. If one of the trends of evolution is that more of the life span is to be spent in gainful waking activity, then surely these people are in the van of this advance.
Questions 21 - 25 are based on the passage you have just heard.
21. What does the author seem to indicate?
22. Why does the author disagree with Dr. Burton?
23. What does the author point out?
24. What do we learn from the passage?
25. According to the author, why some people are alert at bedtime and sleepy when they get up?