A、The longer our nap is, the more energy we will get. B、A 20-minute nap will help to restore our energy. C、A short-time nap does

Medical experts say most Americans do not get enough sleep. They say more Americans need to rest for a short period in the middle of the day. They are advising people to sleep lightly before continuing with other activities. (13)One study earlier this year found that persons who sleep for a few minutes during the day were less likely to die of heart disease. The study followed more than 23 000 Greek adults for about six years. Adults who rested for half an hour at least three times a week had a 37% lower risk of dying from heart disease than those who did not nap. Study organizers said the strongest evidence was in working men. The organizers said naps might improve health by reducing tension caused by work. Some European and Latin American businesses have supported the idea of napping for many years. They urge people to leave work, go home and have a nap before returning. In the United States, some companies let workers rest briefly in their offices. (14)They believe this reduces mistakes and accidents, and also increases the amount of work a person can do. Sleep experts say it’s likely that people make more mistakes at work than at other times. They say people should not carry out important duties when they feel sleepy. And they say the best thing to do is to take a nap. (15)About 20 minutes of rest is all you need. Experts say this provides extra energy and can increase your effectiveness until the end of the day. But experts warn that a nap should last no more than 20 to 30 minutes. A longer nap will put the body into deep sleep. Waking up will be difficult.
   Questions 13 to 15 are based on the passage you have just heard.
   13. What does the study suggest according to the passage?
   14. Why do some companies encourage their workers to take naps?
   15. Which of the following may sleep experts agree with?