听力原文：W: We've all heard the saying 'Laughter is the best medicine.' How important is it to out health, Dr. Berk? M: This saying has been scientifically proven! According to our research, laughter strengthens the immune system and lowers the levels of stress hormones... W: Can you be more specific? M: Well, (23)after a bout of laughter, blood pressure drops to a lower, healthier level than when the laughter began. Dr. William Fry of Stanford University found that 'twenty seconds of guffawing gives the heart the same workout as three minutes of hard rowing'. W: What else? M: Another year-long study of heart attack victims done at the Oakhurst Health Research Institute in California found that of those patients who spent half an hour a day watching comedy videos, 10 percent had a second heart attack, whereas 30 percent of those who did not watch had a second attack. W: Wow! Laughter is really a good medicine to patients. So do we have to learn to laugh? M: Not necessarily. (24)Since laughing is something people can do sitting down, costs no money, and requires no special exercise equipment or skill, it's the perfect workout for anyone who doesn't have the time or desire to participate in a regular fitness program. W: Oh, I see. What about the situation in which you aren't in a mood to laugh? M: OK, remember, (25)even when you fake a smile or laugh, you get the same physiological benefits as when it's the real thing, because your mind is smart, but your body is stupid and can't tel1 the difference. W: Great! I've learned it. Thank you very much! M: My pleasure. (20)