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?
A．The effects of. caffeine.
B．Some causes of headaches.
C．How to do well on exams.
D．Problems with the student cafeteria.

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