听力原文：W: Kevin, You are having another cup of coffee? That's your third since lunch. M: (20)Yeah. Well, I stayed up all night reviewing for my history exam. I couldn't keep my eyes open in my last class. I'm having this coffee so I can stay awake this afternoon. W: Don't you know that drinking too much coffee is harmful? M: What do you mean harmful? W: Well, for one thing it may cause heart disease. M: (21)But most of the research about the link between coffee and the heart disease is not so persuasive. The conclusion is not so definite. W: But coffee has caffeine, which is addictive. M: You can't become addicted to caffeine like other drugs. W: En, you can. Don't you know that people who are deprived of caffeine suffer with symptoms, especially, headaches? M: One cure for headaches, oddly enough, is caffeine. Haven't you ever noticed that many drugs for curing headache use caffeine as one of their ingredients? Besides, coffee helps me work faster. W: (22)Studies have shown that coffee makes you work faster but not necessarily better. You may finish your exam in a shorter period of time but you won't have fewer errors. M: You know, that Coca-Cola you are drinking has caffeine in it, too. W: But 12 ounces of coke has only half the caffeine of a 5 ounces cup of coffee. And a cup of tea has less than that. M: But I like the taste of coffee. W: You could drink decal, which has much less caffeine. M: Decal doesn't have as much flavor as regular coffee. Besides I want something that will keep me alert in class. W: Well, you are stubborn. (23)