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OFFICIAL50 Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it. To review the passage, click VIEW TEXT. Stars generate the energy that makes them shine as a by-product of nuclear fusion and not by shrinking, as scientists had once believed

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Until the early- to mid-twentieth century, scientists believed that stars generate energy by shrinking. As stars contracted, it was thought, they would get hotter and hotter, giving off light in the process. This could not be the primary way that stars shine, however. If it were, they would scarcely last a million years, rather than the billions of years in age that we know they are. We now know that stars are fueled by nuclear fusion. Each time fusion takes place, energy is released as a by-product. This energy, expelled into space, is what we see as starlight. The fusion process begins when two hydrogen nuclei smash together to form a particle called the deuteron (a combination of a positive proton and a neutral neutron). Deuterons readily combine with additional protons to form helium. Helium, in turn, can fuse together to form heavier elements, such as carbon. In a typical star, merger after merger takes place until significant quantities of heavy elements are built up.

We must distinguish, at this point, between two different stellar types: Population I and Population ll, the latter being much older than the former. These groups can also be distinguished by their locations. Our galaxy, the Milky Way, is shaped like a flat disk surrounding a central bulge. Whereas Population I stars are found mainly in the galactic disk, Population II stars mostly reside in the central bulge of the galaxy and in the halo surrounding this bulge.

Population II stars date to the early stages of the universe. Formed when the cosmos was filled with hydrogen and helium gases, they initially contained virtually no heavy elements. They shine until their fusible material is exhausted. When Population II stars die, their material is spread out into space. Some of this dust is eventually incorporated into newly formed Population I stars. Though Population I stars consist mostly of hydrogen and helium gas, they also contain heavy elements (heavier than helium), which comprise about 1 or 2 percent of their mass. These heavier materials are fused from the lighter elements that the stars have collected. Thus, Population I stars contain material that once belonged to stars from previous generations. The Sun is a good example of a Population I star.

What will happen when the Sun dies? In several billion years, our mother star will burn much brighter. It will expend more and more of its nuclear fuel, until little is left of its original hydrogen. Then, at some point in the far future, all nuclear reactions in the Sun’s center will cease.

Once the Sun passes into its "postnuclear" phase, it will separate effectively into two different regions: an inner zone and an outer zone. While no more hydrogen fuel will remain in the inner zone, there will be a small amount left in the outer zone. Rapidly, changes will begin to take place that will serve to tear the Sun apart. The inner zone, its nuclear fires no longer burning, will begin to collapse under the influence of its own weight and will contract into a tiny hot core, dense and dim. An opposite fate will await the outer region, a loosely held-together ball of gas. A shock wave caused by the inner zone's contraction will send ripples through the dying star, pushing the stellar exterior's material farther and farther outward. The outer envelope will then grow rapidly, increasing, in a short interval, hundreds of times in size. As it expands, it will cool down by thousands of degrees. Eventually, the Sun will become a red giant star, cool and bright. It will be so large that it will occupy the whole space that used to be the Earth's orbit and so brilliant that it would be able to be seen with the naked eye thousands of light-years away. It will exist that way for millions of years, gradually releasing the material of its outer envelope into space. Finally, nothing will be left of the gaseous exterior of the Sun; all that will remain will be the hot, white core. The Sun will have become a white dwarf star. The core will shrink, giving off the last of its energy, and the Sun will finally die.

14.Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it. To review the passage, click VIEW TEXT. Stars generate the energy that makes them shine as a by-product of nuclear fusion and not by shrinking, as scientists had once believed

A.The Sun is a good example of a Population I star because the Sun generates its energy through nuclear fusion rather than through contraction

B.In the Milky Way, Population I stars are found in and around the central bulge and Population II stars are found in the galactic disk

C.The Sun and stars like it will separate into inner cores and outer envelopes before all nuclear reactions in the cores stop and the stars finally die

D.Population II stars, the oldest stars, are formed from hydrogen and helium gases, and they shine until they exhaust their fusible material

E.Population I stars, including the Sun, are relatively young stars that are mostly hydrogen and helium gas but also contain heavier elements

F.The outer envelope of the Sun and stars like it will release their energy into space, and the inner cores will become white dwarfs before they finally give off their last energy

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正确答案:DEF
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【题目翻译】:下面提供了关于该段落简要概述的介绍性句子。 通过选择表达该段落中最重要想法的三个答案选项来完成摘要。 有些句子不属于摘要,因为它们表达了段落中没有提出的想法,或者是段落中的小想法。 这个问题值得2分。 将答案选项拖到它们所属的空间。 要删除答案选项,请单击它。 要查看该段落,请单击“查看文本”。 正如科学家曾经认为的那样,恒星产生的能量使它们成为核聚变的副产品,而不是通过萎缩 A: 太阳是一个人口明星的一个很好的例子,因为太阳通过核聚变而不是通过收缩产生能量 B:在银河系中,人口I恒星位于中央凸起和周围,人群II恒星位于银河盘中 C:在核心中的所有核反应停止并且恒星最终死亡之前,太阳和星星将会分离成内核和外封套 D:人口II恒星,最古老的恒星,由氢气和氦气形成,它们会发光,直到它们排出可熔材料 E:包括太阳在内的人群I星是相对年轻的恒星,主要是氢气和氦气,但也含有较重的元素 F:太阳和外星的外壳像它会将能量释放到太空中,内核将变成白矮星,然后它们最终释放出最后的能量 【判定题型】:根据问题的提问方式和7选5的作答方式可以确定该题目为表格题。 【选项定位及分析】: A选项说太阳是星族I恒星的一个例子,因为它通过核聚变产生能量,而不是通过收缩产生能量。首先,后半句话是错误的,对应文章第三段,区分星族I和星族II的关键,一是恒星年龄,而是恒星位置,和是否通过核聚变产生能量无关。其次,这句话只是一个举例,属于细枝末节信息,不是文章概要,故排除。 B选项说银河系中,星族I的恒星处于中心凸起的位置或周围,而星族II的恒星处于扁平的星盘上。这句话与第二段的最后一句话直接矛盾,故B排除。 C选项说太阳和与它相似的恒星,会先分裂成内部和外部,然后内核的核聚变会停止,最终恒星会死亡。对应文章第四、第五段。但是在第四段末提到“Then, at some point in the far future, all nuclear reactions in the Sun’s center will cease.”然后第五段才开始描述太阳分为内部和外部的情况,所以说是核聚变停止在先,然后太阳才开始分为2个部分。C选项时间顺序颠倒,故排除。 D选项描述星族II的恒星,对应文章第三段1~3句,符合原文内容,故为正确答案。 E选项描述星族I的恒星,对应文章第二、三段的内容,符合原文,故为正确选项。 F选项对应文章第五段,描述太阳变成红巨星之后的过程,外部会先释放完所有物质,然后内核变成白矮星,在释放完内核最后的能量之后,太阳会死亡。F选项符合原文,故为正确答案。

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