A、By crushing large amounts of stone. B、By causing mud or sand to mix with ocean water. C、By creating large waves on the surface

W: Dr. Smith. I need to ask you about something in Chapter 12. Well, it talks about erosion and the deposit of sediment（solid matter that settles at the bottom of a liquid）on the deep sea floor, but I’m confused. For that to happen, there would have to be some kind of force down there moving the mud or sand or whatever, right? But I thought there weren’t any waves or currents that deep in the ocean.
M: That’s not exactly correct. It’s true that deep down on the ocean floor we won’t have the same kinds of currents we find in shallower parts. But we do find what we call turbidity currents. That wasn’t in the book but the term came up in class. Do you remember what it means?
W: Um, turbidity currents? Oh, yeah! Isn’t that when sediment like sand or mud gets stirred up and mixes with water? And then this mixture flows through the water around it, right?
M: Good!
W: Uh, but I thought that only happened in rivers.
M: Well, yes. This does occur in rivers, but we see turbidity currents deep in the ocean as well.
W: But how do they get started there?
M: Earthquakes mostly. When an earthquake occurs under water, it throws up tremendous amounts of mud or sand that becomes suspended in a layer of water near the bottom of the ocean. This layer is so much denser and heavier than other ocean water that it flows right down the slopes of the ocean floor and gains more and more speed as it moves along. Then it’s just like the winds blowing across the desert picking up sand from one place and moving it along and finally dropping it somewhere else.
Questions 19 to 22 are based on the conversation you have just heard.
19. What is the main topic of the discussion?
20. What does the professor ask the student to explain?
21. According to the professor, how do earthquakes cause turbidity currents?
22. Why does the professor mention desert winds?