Questions are based on the recording you have just heard. __________

单选题Questions are based on the recording you have just heard.
______________

A．The difference in age among American mountain ranges.
B．The importance of a technique used for dating.
C．The recent discoveries about a mountain range.
D．A comparison of various minerals used for dating.

参考答案B

解析:[22] Today I want to talk about a way in which we are able to determine how old a piece of land. or some other geologic feature is -- dating techniques. I'm going to talk about a particular dating technique. Why? Good dating is a key to good analysis. In other words, if you want to know how a land formation was formed, the first thing you probably want to know is how old it is. It's fundamental.
Um... Take the Grand Canyon for instance. Now, we geologists thought we had a pretty good idea of how the Grand Canyon in the southwestern United States was formed. Essentially it was part of a vast desert.[23] And until just recently, most of us thought the sand had come from an ancient mountain range fairly close by that flattened out over time. That's been the conventional wisdom among geologists for quite some time. But now we've learned something different, and quite surprising, using a technique called Uranium-Lead Dating. I should say that Uranium-Lead Dating has been around for quite a while. But there have been some recent improvements. Anyway, Uranium-Lead Dating has produced some surprises. Two geologists discovered that about half of the sand from the Grand Canyon was actually once part of the Appalachian Mountains. That's really eye-opening news, since the Appalachian Mountain Range is, of course, thousands of kilometers to the east of the Grand Canyon.
Of course, the obvious question is how did that sand end up so far west? The theory is that huge rivers and wind carried the sand west where it mixed in with the sand that was already there. Well, that was a pretty revolutionary finding. Uranium-Lead Dating allowed them to go about it in an entirely different way. So because of this, this new way of doing Uranium-Lead Dating, we've been able to determine that one of our major assumptions about the Grand Canyon was wrong. Like I said before, Uranium-Lead Dating has been with us for a while. [24] However it took a long time before you got results. It just wasn't very efficient. And it wasn't very accurate. But technical advances have cut down on the number of grains you have to study, so you get your results faster. So I'll predict that Uranium-Lead Dating is going to become an increasingly popular dating method.
[25] There are a few pretty exciting possibilities for Uranium-Lead Dating. Here is one that comes to mind. You know the theory that earth's continents were once.joined together and only split apart relatively recently?Well, with Uranium-Lead Dating, we could prove that more conclusively. If they show evidence of once having been joined, that could really tell us a lot about the early history of the planet's geology.
【预测】选项均为概括性较强的名词短语表明，本题可能考查录音的主旨。
【解析】B。教授一开始就介绍今天将讨论一种年代测定技术，接着通过举例侧重说明了铀一铅测定技术(Uranium-Lead Dating.在现实研究中的重要性，故答案为B。

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单选题 ________________

A．A desert that once connected two continents.
B．An ancient lake located in the American Southwest.
C．Sands carried by a river from the Appalachian Mountains.
D．A nearby mountain range that had flattened out over time.

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单选题 ________________

A．It can be performed outside a laboratory.
B．It can now be done more efficiently.
C．It no longer involves radioactive elements.
D．It can be used in fields other than geology.

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单选题 ________________

A．To explain how the Grand Canyon was formed.
B．To demonstrate how difficult the dating technique is.
C．To disprove a theory about the age of Earth's first mountain ranges.
D．To give another example of how the dating technique might be useful.

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