Question 20 is based on the passage you have just heard.

    The thought of having no sleep for 24 hours or more isn't a pleasant one for most people. The amount of sleep that each person needs varies. In general, each of us needs about 8 hours of sleep each day to keep us healthy and happy. Some people, however, can get by with just a few hours of sleep at night.

    It doesn't matter when or how much a person sleeps. (19) But everyone needs some rest to stay alive. Few doctors would have thought that there might be an exception to this. Sleep is, after all, a very basic need. (20) But a man named Al Herpin turned out to be a real exception, for supposedly, he never slept!

    Al Herpin was 90 years old when doctors came to his home in New Jersy. They hoped to challenge the claim that he never slept. But they were surprised. Though they watched him every hour of the day, they never saw Herpin sleeping. He did not even own a bed. He never needed one.

    The closest that Herpin came to resting was to sit in a rocking chair and read a half dozen newspapers. His doctors were puzzled by the strange case of permanent sleeplessness. (21) Herpin offered the only clue to his condition. He remembered some talk about his mother having been injured several days before he had been born. Herpin died at the age of 94, never, it seems, having slept at all.

(20). What do doctors think of Al Herpin's case?

解析：细节理解题。根据文章第二段中“But a man named Al Herpin turned out to be a real exception, for supposedly, he never slept!”可知，Al Herpin从不睡觉，是个特例。因此选C项。