A、Every species becomes extinct. B、Humans beings are powerful enough to kill other species. C、The population of human beings is

W: Professor Marnes, 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 crux 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: 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 are also helped by others. For example, the common housefly and cockroaches 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: To a certain extent, humans were initially lucky enough to have the right weather conditions and a lack of predators, but now, of course, we survive by ingenuity!
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 not 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.
Questions 19 to 22 are based on the conversation you have just heard.
19. Why did the woman go to see the professor?
20. What puzzles the woman about extinction?
21. Why is the professor not so sure that humans will never become extinct?
22. What is inferred by the professor at the end of the lecture?