21天搞定文本，即《出国留学备考系列:21天搞定新托福听力》将21天的训练划分为5个章节21天，每个章节对应一周的时间，每一天的训练包含4个填空训练和5个听写训练。今天21天搞定托福听力 pdf文本节选，供同学们参考学习，助力大家听力备考！
B: 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(混浊流). This wasn’t in the book but the term came up in class. Do you remember what it means?
A: Um, turbidity currents? Oh, yeah! Isn't that when sediment like sand or mud gets stirred up and mixes with water，and then this mixer kind of flows through the water around it, right?
B: Good!
A: Um, but I thought that only happened in rivers.
B: Well, yes. This does occur in rivers, but we see turbidity currents deep in the ocean as well. A: But how do they get started there?
B: 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 if 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.
31：What is the main topic of the discussion? (B) The movement of sediment deep in the ocean.
32：What does the professor ask the student to explain? (D) What a turbidity current is.
33：How is a turbidity current different from the water around it? (B) it's heavier.
34：According to the professor, how do earthquakes cause turbidity currents? (C) By causing mud or sand to mix with ocean water
35：Why does the professor mention desert winds? (C) To explain the effects of turbidity currents.
A: Hey Neal, now that the midterms are over a bunch of us are getting away for the weekend to go canoeing. Wanna come along'? B: Well, um, it'd be great to get away, but I've never done it before.
A: None of the others have either except for me. I went once last fall. But there'll be an instructor in each canoe the first day. B: I don't know.
A: Oh, come on. This is our last chance to take a break before finals. The scenery is beautiful, and if it gets too hot we can dive in whenever we feel like it. The river's really calm this time of the year, no rapids(急流) to deal with.
B: That's a relief. What would I have to bring?
A: Let's see. Tom's bringing food for the Friday night cookou(t郊游野餐)for everyone. And the people who run the trip have tents set
up and they supply food and drinks for all day Saturday. On the way back Sunday morning we'll stop somewhere for breakfast. So, you just have to bring a bathing suit(泳衣) and a sleeping hag.
B: Well, I do love camping and sleeping out. Where is this place?
A: Well, it's about an hour and a half to the place where we meet the trip leaders. We leave our car there and they drive us and the canoes upriver to the place where we start canoeing.
B: And who's driving us to the meeting place?
A: Well, I was hoping we could take your car. Mine's in the shop again.
B: Oh, I see. It's not me you want, it's my car. A: Don't be silly. So what would you say?
B: Oh, why not!
A: Great I'll give you a call after I find out when everyone wants to leave on Friday.
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