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 orbits of comets.
B．The structure of comets.
C．The origin and life cycle of comets.
D．Unique characteristics of a famous comet.

参考答案A

解析:OK. We have been looking at some of the smaller members of our solar system, comets. You already know about the structure of comets. [16] Let's continue our dis-cussion now by talking about orbits, especially those of the so-called periodic-orbit comets. These are the comets that circle around the Sun pretty regularly. They return again and again, predictably, after a certain period of time.That's why we say their orbits are periodic. Probably the most famous and brightest of these is Halley' s Comet.
Halley' s Comet comes from far out in the solar system, goes in close to the Sun, and then out again. At its closest approach to the Sun, Halley' s Comet is about twice as close to the Sun as Earth is. And at its farthest. It's about thirty-five times farther from the Sun than we are, which puts it out beyond Neptune. With Halley' s Comet, well, it returns every 75 years, roughly.
But where is Halley' s Comet during most of this time? Well, like all orbiting bodies, a comet moves faster when it's closer to the Sun. So it only spends about a year or two in our neighborhood, inside the orbit of Jupiter. [17]Most of its time is spent way out beyond Jupiter's orbit,poking along near the farther reaches of its own orbit. Be-cause of this, we can only see Halley' s for a few months every 75 years, first on its way in toward the Sun, and then on its way out again.
Now, you remember from our previous discussion that a comet's nucleus, its core, is made up of ice and dust,like a frozen snowball. [18] And as it approaches the Sun,it starts to heat up. And some of the ice tunas into gas and spreads out from the nucleus. The gases, the comet never collects them back again, so on every orbit,the comet leaves part of itself behind.
OK. How old is this solar system? Four and a half billion years, remember? And Halley's is going around the Sun once every 75 years and losing stuffeach time. So the comet should be long gone by now, right? I mean, how come Halley's is still there? After four and a half billion years. How could it be? [19] Well, the answer is that this comet hasn't always been in such a short periodic orbit,since once a comet gets into an orbit that keeps it coming in close to the Sun quite frequently. Well, that comet's probably not going to be around too much longer. So this kind of periodic orbit is only a phase in a comet's life.A phase that just precedes its final breakup. We've seen comets do that, going toward the Sun and then come back around, tom into pieces.
【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】A。教授一开始就提出了本次讲话的主要内容——继续关于彗星的探讨，尤其是那些所谓的周期轨道彗星(periodic-orbit comets)，之后以哈雷彗星为例具体介绍了周期彗星的相关知识。A是对本录音内容的高度概括，故为答案。

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单选题 ______________

A．To account for the comet's composition of ice and dust.
B．To show the effect of Jupiter's gravity on the comet's orbit.
C．To explain why the comet is not visible from Earth very often.
D．To argue that the comet's orbit should not be considered periodic.

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单选题______________

A．It moves slower.
B．It starts to heat up.
C．It loses some material.
D．It absorbs gases from the Sun.

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单选题 ______________

A．It has always had the same orbit it has now.
B．It used to be much smaller than it is now.
C．It did not always have the same orbit it has now.
D．It is much younger than the rest of the solar system.

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