【正确答案】 A

【答案解析】[听力原文]
   W: I can't believe my eyes, Bill. Are you having a salad for lunch? No hamburger?
   M: Get used to it, Susan. I'm off burgers for good. Most fast food is loaded with Na—a kind of chemical substance. I've been reading about a new healthy snack food. It's made from fish. Have you heard anything about it?
   W: No, I haven't. But I don't like fish very much anyway. But why are you interested in lessening your salt intake?
   M: My doctor says I should cut back on Na-rich foods to lower my blood pressure. Actually, my blood pressure isn't that high, but they say it increases with age.
   W: I would have thought you were too young for that. Well, I'm glad you're starting to pay more attention to your diet.
   M: I've been eating lots of fruits and vegetables recently. I want to lower my risk of a heart attack and stroke, too. Recently, my family are trying to eat more healthy food.
   W: So am I. That's why I've been trying to lower my cholesterol (胆固醇) level. Just like you, I'm now trying to lose a few pounds, too.
   M: In that case, you should come to my health club with me. Can you tear yourself away from the TV tonight?
   W: You've joined a health club? I would never have dreamed that you'd take something like this so seriously. Why not? If you can break your unhealthy eating habits, I can surely break a sweat.
   M: I can do 100 push-ups in a row. Don't get jealous. You will do it someday.
   Which substance can be found in most fast food according to the man?