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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

admin2013-05-04  64
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.
We can conclude that this article is intended for ______.
选项 A、general readers
B、scientists
C、students majoring in physics
D、astronomers
答案A
解析 用排除法解。这是一篇科普性质的文章，读者对象不可能是科学家(从大量名词解释可以看出)，也不是针对宇航员的而是关于他们的报道；若是针对物理专业的学生，话题义显得浅显，而且又谈了天文方面的知识，所以只能是一般读者。
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