Astronomers now have afairlygood idea of what acometreally is. When it is far from the Sun, it is a very small object only a few kilometersacross. 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,radiationfrom the Sun vaporizes the icy matter and releases some of the dust. This forms agigantichalo around the ice ball. This halo-called thecomaextends out tens of thousands of kilometers from the icycore, which is thenucleusof thecomet. Sunlight reflected off the dust particles makes thecomavisibleto observers on Earth. Ultravioletradiationfrom the Sun breaks down thevapormolecules into their constituents. These components can be excited by absorbingradiationfrom the Sun. In returning to lower-energy states, the excited atoms and ionsemitlight, contributing to theluminosityof thecoma.

When thecometgetsevencloser to the Sun, one of its mostspectacularparts begins to form-the tail. Actually, there are two kinds of tails: the dust tail and theiontail. The dust tail is produced by the light from the Sun reflecting off the dust particles in thecoma. A photon carriesmomentum. In bouncing off a dustparticle, it imparts a tiny, but perceptible,momentumchange to the dustparticle, driving it away from thecoma. As thecometsweepsalongitsorbit, it leaves a curvingtrailof dust behind in itspath. Thisvisibledust tail canextendfor tens or hundreds of millions of kilometers out from thenucleus. The dust tail is characterized by its gently curvingshapeand its yellowish color.

A differentmechanismisresponsiblefor theiontail. Near the Sun,ultravioletradiationfrom the Sun (solarwind) ionizes and excites the atoms in thecoma. As thesolarwind sweepsthroughthecoma, the high-velocity charged particles of thesolarwindinteractwith the electrically charged excited ions in thecoma, driving them away from the head of thecomet. In returning to lower-energy states, these excited ionsemitphotons and form aluminous,bluish-colored tail extending out from thecometdirectly away from the Sun. Since both kinds of tails are produced byradiationstreaming out from the Sun, theyextendout from thecomain the general direction away from the Sun. Acometmayexhibitseveral tails of each kind.

Although thenucleusis of the order of a few kilometers in size, thediameterof thecomamay be tens or hundreds of thousands of kilometers; the tails typicallyextendout tens or hundreds of millions of kilometers away from thecoma.

Acometleaves atrailof matter behind it as it movesthroughtheinnersolarsystem. Some of thisdebrismay get strewnacrossEarth'sorbitaround the Sun. When Earth passesthroughthis part of itsannualpath, it sweepsthroughthe dusttrail. The particles enter Earth'satmosphereat highvelocity. Theairfrictioncan cause one of these bits of matter to produce abriefstreak of light as it burns up in theatmosphere.

Since acometloses matter on each pass by the Sun,eventuallyit will bedepletedto the point where it is no longervisible. Comets thatapproachthe Sun havefinitelifetimes. Giventhetypicalsizes of comets and thetypicalrates 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 thesolarsystem. Where do the new comets come from toreplacethe old ones that dissipate andvanishfrom view?

DutchastronomerJan Oort proposed that agiantcloud of matter left over from theformationof thesolarsystem surrounds the Sun and extends out to about 50,000astronomicalunits. This cloud contains large chunks of matter like thenucleiof comets. Thegravitationalinfluence of a passing star can besufficientto perturb theorbitof one of these chunks to send it toward theinnersolarsystem and bring it near the Sun.

Questions:
question_stem: The word “imparts” in the passage is closest in meaning to
choices: tag: A
text: shows

tag: B
text: gives

tag: C
text: confirms

tag: D
text: maintains
answer_key: B
explanation: impart v. 把（某性质）赋予。
与 B 选项 give 同义。
故选 B。

question_stem: According to paragraphs 2 and 3, all of the following statements about comet tails are true EXCEPT:
choices: tag: A
text: Ion tails are created by ultraviolet radiation.

tag: B
text: Photons are active in creating both ion tails and dust tails.

tag: C
text: Dust tails follow in the path of a comet.

tag: D
text: Ion tails have a curved shape and yellowish color.
answer_key: D
explanation: 题干提问“除了以下哪个选项之外，剩余描述彗尾的选项都是正确的”，建议先看完四个选项，再逐一对应文本。
定位第3段第1-2句，对应 A 选项，正确；
定位第2段第4句及第3段第4句，均提及 photons，对应 B 选项，正确；
定位第2段第6句，对应 C 选项，正确；
定位第2段最后一句，its gently curving shape and its yellowish color 修饰 dust tail，再根据第3段第4句 a luminous, bluish-colored tail 修饰 ion tail，D 选项描述 ion tails 是 yellowish color，错误。
故选 D。

question_stem: According to paragraph 3, the tails of a comet
choices: tag: A
text: flow in the same direction as the radiation that produces them

tag: B
text: become straighter as the comet moves away from the Sun

tag: C
text: are pulled toward the Sun

tag: D
text: can become detached from the comet head if the solar winds are strong enough
answer_key: A
explanation: 题干提问“彗星的尾巴”。
定位第3段倒数第2句 they extend out from the coma in the general direction away from the Sun，远离太阳的方向，对应 A 选项。
故选 A。

question_stem: Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
choices: tag: A
text: The diameter of a coma is usually larger than the tails that extend away from it, while the nucleus is much smaller.

tag: B
text: The diameters of the nucleus, coma, and tail of a comet exhibit minor differences in size.

tag: C
text: The nucleus is reduced in size, while the coma and tails extend far into space.

tag: D
text: Comet tails are much larger than the coma, which is much larger than the nucleus.
answer_key: D
explanation: 第4段高亮句主干：Although the nucleus is a few kilometers， the diameter of the coma may be tens or hundreds of thousands of kilometers; the tails extend out tens or hundreds of millions of kilometers.
A 选项 larger 比较说反，应该是 tail 比 the diameter of the coma 大，错误；
B 选项 exhibit minor differences in size 不符合原文，错误；
C 选项 The nucleus is reduced in size 未提及，错误；
D 选项 tail > coma, coma > necleus，正确。
故选 D。

question_stem: According to paragraph 5, what happens when Earth passes through the dust trail of a comet?
choices: tag: A
text: Dust particles bum up in Earth's atmosphere.

tag: B
text: The comet's orbit is changed.

tag: C
text: The dust trail is completely destroyed.

tag: D
text: The comet’s tails change direction.
answer_key: A
explanation: 题干提问“当地球经过彗星的尘埃尾时发生了什么？”
定位第5段第3-5句，根据第5句 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. 为所发生事件，其中 produce a brief streak of light as it burns up in the atmosphere 对应 A 选项。
故选 A。

question_stem: What does paragraph 6 imply about comets that are visible from Earth?
choices: tag: A
text: The visible comets grow brighter as they grow older.

tag: B
text: The comets that are visible from Earth today have not been orbiting close to the Sun throughout the history of the solar system.

tag: C
text: There were fewer visible comets during the early history of the solar system than there are today.

tag: D
text: The average lifetime of the visible comets is decreasing.
answer_key: B
explanation: 题干提问“在地球上能够看得到的彗星”。
定位第6段第3句 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，理解为 comet 比 solar system 年龄小，即在 solar system 形成时还没有 comet，后来才有了 comet，对应 B 选项。
故选 B。

question_stem: According to paragraph 7, how did astronomer Jan Oort contribute to our understanding of comets?
choices: tag: A
text: He theorized that comets form when passing stars push chunks of debris from the outer to the inner solar system.

tag: B
text: He proposed a way of comparing comets in our own solar system with those that orbit other stars.

tag: C
text: He explained that the lifetime of a comet depends on its distance from the Sun at the time the solar system was formed.

tag: D
text: He proved that old comets do not really disappear; they just move beyond the limits of the solar system.
answer_key: A
explanation: 题干提问“人们对彗星的理解，Jan Oort 做出了什么贡献”。
定位第7段最后一句 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. 对应 A 选项。
故选 A。