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  3. Mounting evidence has led astronomers to just such an improbable conclusion, at least 90 percent and possibly 99 percent of all

Mounting evidence has led astronomers to just such an improbable conclusion, at least 90 percent and possibly 99 percent of all

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问题     Mounting evidence has led astronomers to just such an improbable conclusion, at least 90 percent and possibly 99 percent of all matter in the universe is completely invisible.  Astronomers call this invisible stuff dark matter.
    An astronomer first reported the invisible matter after noting some unusual behavior in visible matter. In 1993, Swiss astrophysicist Fritz Zwicky observed that galaxies in a cluster of galaxies travelled at higher speeds than expected. He concluded that something was causing strange gravitational effects on these galaxies.
    Galaxies are like vast islands in deep space containing billions of stars that are held together by the force of gravity. Some galaxies are part of larger systems called clusters, which contain many galaxies. Most galaxies, however, belong to smaller systems called groups. Groups and clusters in turn form even larger structures known as superclusters. Just as the force of gravity keeps stars together in galaxies, so the force of gravity appears to keep galaxies together in clusters.
    This same force of gravity keeps planets orbiting around the sun, rather than flying off into space. How great a force gravity exerts depends on the mass of the objects attracted and the distance between them, a principle discovered by Newton. The amount of mass and the distance between objects also determine how fast the objects travel as they orbit each other. For example, in our solar system, the sun is the most massive object, making up about 99.9 percent of the mass of the solar system, and the force of its gravity keeps Earth and the other planets in orbit around it. The orbital speeds of the planets decline with increasing distance from the sun. The planet nearest to the sun, Mercury, travels at an orbital speed of 48 kilometers per second, while the distant planet Pluto orbits at speed of 4.7 kilometers per second.
In astronomical terms, a cluster ______.
选项 A、consists of galaxies
B、makes up superclusters
C、consists of groups
D、makes up a group
答案A
解析 第三段对几个概念进行了说明：星系(galaxy)由星星组成，几个星系组成一个星团 (cluster)，规模小的几个星系组成星群(group)，而星团、星群又组成更大的超星系团。星群与星团不是个体与整体的关系，二者的区别仅在于其大小而已。
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