托福 托福 63 - Structure and Composition of Comets
题目
1 2 3 4 5 6 7 8 9 10 11 12 13 14
11.Why does the author end paragraph 6 with a question?
  • A.To indicate that certain information about comets is unknown
  • B.To introduce a topic that will be clarified in the next paragraph
  • C.To propose an area that requires further research
  • D.To challenge theories that were presented earlier in the passage
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    答案解析:
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    Astronomers now have a fairly good idea of what a comet really is. When it is far from the Sun, it is a very small object only a few kilometers across It consists mainly of ices (water, methane, ammonia) with bits of dust embedded in it-a kind of dirty ice ball. As it approaches the Sun, radiation from the Sun vaporizes the icy matter and releases some of the dust. This forms a gigantic halo around the ice ball. This halo-called the coma-extends out tens of thousands of kilometers from the icy core, which is the nucleus of the comet. Sunlight reflected off the dust particles makes the coma visible to observers on Earth. Ultraviolet radiation from the Sun breaks down the vapor molecules into their constituents. These components can be excited by absorbing radiation from the Sun. In returning to lower-energy states, the excited atoms and ions emit light, contributing to the luminosity of the coma.

    When the comet gets even closer to the Sun, one of its most spectacular parts begins to form the tail. Actually, there are two kinds of tails the dust tail and the ion tail. The dust tail is produced by the light from the Sun reflecting off the dust particles in the coma. A photon carries momentum. In bouncing off a dust particle, it imparts a tiny, but perceptible, momentum change to the dust particle, driving it away from the coma. As the comet sweeps along its orbit, it leaves a curving trail of dust behind in its path. This visible dust tail can extend for tens or hundreds of millions of kilometers out from the nucleus. The dust tail is characterized by its gently curving shape and its yellowish color.

    A different mechanism is responsible for the ion tail. Near the Sun, ultraviolet radiation from the Sun (solar wind) ionizes and excites the atoms in the coma. As the solar wind sweeps through the coma, the high-velocity charged particles of the solar wind interact with the electrically charged excited ions in the coma, driving them away from the head of the comet. In returning to lower-energy states, these excited ions emit photons and form a luminous, bluish-colored tail extending out from the comet directly away from the Sun. Since both kinds of tails are produced by radiation streaming out from the Sun, they extend out from the coma in the general direction away from the Sun. A comet may exhbit several tails of each kind.

    Although the nucleus is of the order of a few kilometers in size, the diameter of the coma may be tens or hundreds of thousands of kilometers, the tails typically extend out tens or hundreds of millions of kilometers away from the coma.

    A comet leaves a trail of matter behind it as it moves through the inner solar system. Some of this debris may get strewn across Earth's orbit around the Sun. When Earth passes through this part of its annual path, it sweeps through the dust trail. The particles enter Earth's atmosphere at high velocity. The air friction can cause one of these bits of matter to produce a brief streak of light as it burns up in the atmosphere.

    Since a comet loses matter on each pass by the Sun, eventually it will be depleted to the point where it is no longer visible. Comets that approach the Sun have finite lifetimes. Given the typical sizes of comets and the typical rates at which they lose matter, astronomers have concluded that the lifetimes of comets with orbits that bring them near enough to the Sun to be seen from Earth are very much shorter than the age of the solar system. Where do the new comets come from to replace the old ones that dissipate and vanish from view?

    Dutch astronomer Jan Oort proposed that a giant cloud of matter left over from the formation of the solar system surrounds the Sun and extends out to about 50,000 astronomical units. This cloud contains large chunks of matter like the nuclei of comets. The gravitational influence of a passing star can be sufficient to perturb the orbit of one of these chunks to send it toward the inner solar system and bring it near the Sun.

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    最新提问
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      觉得B C 意思一样,不知道选哪个
    • wx_5576
      这道题C为什么对,E为什么不对?
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      B为什么不能选啊?
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      这道题不选E是因为太细节了吗
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      这题为什么选C?
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      小雨淅沥哗啦的下 去解答 去解答
      B哪里错了
    • 小雨淅沥哗啦的下
      小雨淅沥哗啦的下 去解答 去解答
      B为啥不对
    • 李浩然
      B选项错误,是因为残缺么?
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      鑫哥,从哪里看出来这个masks 不是use呀,原文说了wear呀
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      鑫哥,想知道E错在哪里?
    • wx_7695
      鑫哥,B选项 cannot extended to earlier geological periods. 原文说的意思是后来的进化无法估计吧
    • wx_2163
      B为什么不选
    • wx_7780
      鑫哥,这个哪里看不use了。BD是修饰错,C是无中生有,怎么能直接选出A?
    • 100
      看到第一句话,以为是中心句就选了A... 为什么不能选A呢
    • 100
      为什么选b?
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    • 我是啦啦啦
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      这个题A哪里错了?是因为主语不对吗?这个C比A多一步推理啊
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      鑫哥,D选项里的19世纪出现了很多假设,原文中并没有提到啊
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      为什么选a 呢。我觉得a是细节。F哪错了?
    • wx_1105
      我想问一下,这道题为什么不能选A呢?
    • wx_8122
      D为什么不选
    • wx_1655
      f选项哪里说了
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      鑫哥,原文是below经济损害水平,D是一触发经济损害就用,这也对吗?
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    • wx_6697
      鑫哥,这道题答案是不是错了,好多人选D 我也选的D求解答
    • wx_6697
      这道题应该是一道易错题,每个选项的都有,然而我选的A错了,求解
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      鑫哥,这题的C是怎么得到的?B也没有找到啊?难道不是应该选B
    • wx_6697
      鑫哥,还有这个,好多人选A答案是不是错了
    • wx_6697
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      it is difficult to say how far they were intended to be portraits rather than generalized images 这句话怎么理解呢
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    • 此楠楠
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