听力原文：W: Gosh! Fred. Another cup of coffee? That's your third since lunch. M: Yeah. Well, I stayed up all night cramming 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: Nah. Most of the research about the link between coffee and heart disease is inconclusive. W: But coffee has caffeine which is addictive. M: You can't become addicted to caffeine like other drugs. W: Yes, you can. Don't you know that people who are deprived of caffeine suffered withdraw symptoms especially headaches. M: One cure for headaches, oddly enough, is caffeine. Haven't you ever noticed that many over-the-counter headache remedies use caffeine as one of their ingredients? W: Besides, coffee helps me work faster. M: 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 soda you're drinking has caffeine in it too. M: But twelve ounces of soda has only half the caffeine of a five 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 decaf. M: Decaffeinated coffee doesn't have as much flavor or body as regular coffee. Besides, I want something that will keep me alert in class. what is the conversation mainly about?