A、It was by continuous exploitation of the environment. B、It was largely by chance. C、It was because of houseflies. D、It was by

W: Professor Marnes,(19)I wonder if you can fill me in on your lecture last Friday. I had to attend a scholarship award ceremony,
M: Oh well, congratulations. I hope you were rewarded handsomely!
W: Well, every bit helps. So, about your lecture, I understand you were talking about extinctions.
M: Yes. Well, the vital part of my talk was just that we tend to think of extinction as a dramatic event, but most species die out over quite a period of time.
W: (20)Why do they die off? I thought they were continuously improving themselves. Natural selection, I think you once mentioned.
M: Ah, but you see while there is natural competition between the species, what determines which species survive is largely by chance.
W: I don’t get it. Why do species bother competing?
M: Well, there are short-term advantages. But many species also are helped by others. For example, the common housefly might have died off years ago if not for humans.
W: But you’re not saying that humans are so successful merely because of chance?
M: (21)To a certain extent, humans were initially lucky enough to have the right weather conditions and a lack of killers who live on us for food, but now, of course, we survive by cleverness.
W: So we may never become extinct.
M: No, because we may be in a crash course to extinction by our continuous exploitation of the environment. We are a relatively young species and our time is hot yet overdue.
W: But there are 6 billion of us!
M: Yes, and there’re many more houseflies too! Each with the capacity to spread one disease from one person to another in a fast period of time.
19. Why did the woman go to see the professor?
20. What puzzles the woman about extinction?
21. What does the man say about humans’ successful survival?