【正确答案】 D

【答案解析】[听力原文] W: Hi, Dan, you know that lab you missed? You can have my notes. M: Thanks. I appreciate that. W: So how are you feeling? M: Much better now that I began taking an antibiotic. Student health center gave me one, and it's really helpful. You know what amazes me that the human races survive before antibiotics. W: I agree. When my father was a young boy in the 1940's, he got blood poisoning and would have died. But his doctor had heard of this new drug, called penicillin. M: Wow, he was really lucky. And now we have lots of antibiotics that kill bacteria. W: Well, penicillin kills bacteria, but not all antibiotics do. Some are just slowing the bacteria down until our normal immune defenses can finish the job. M: Wow, you are a fund of drug trivia. How do you know all these? W: My mother used to look up all our medicines, prescription and non-prescription. There are lots of books around. It's interesting. What antibiotic are you taking? M: I don't remember. It's on the bottle. I think I'll take a new look at the label and drop by the library to see if they have reference books on medicines. See you in lab tomorrow. What are the speakers mainly discussing?