2. 外语
  3. Mounting evidence has led astronomers to just an improbable conclusion; At least 90 percent and possibly 99 percent of all matte

Mounting evidence has led astronomers to just an improbable conclusion; At least 90 percent and possibly 99 percent of all matte

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问题      Mounting evidence has led astronomers to just an improbable conclusion; At least 90 percent and possibly 99 percent of all matter in the universe is completely【C1】______ . Astronomers call this stuff dark matter.
     An astronomer first reported the matter after noting some unusual behaviour in visible matter. In 1993, Swiss astrophysicist Fritz Zwicky concluded that 【C2】______ was causing strange gravitational effects【C3】______ these galaxies.
     Galaxies are like【C4】______ islands in deep space【C5】______ billions of stars that are held together by the force of【C7】______ .Some galaxies are part of【C6】______ systems called clusters; which contain many galaxies. Most galaxies; belong to smaller systems called groups. Groups and clusters in 【C8】______ form even larger【C9】______ known as super-clusters. Just【C10】______ the force of gravity keeps stars together in galaxies the force of gravity【C11】______to keep galaxies together in clusters~
     This same force of gravity keeps planets【C12】______ around the sun【C13】______ flying off into space. How great a force gravity exerts depends on the mass of the objects【C14】______ and the distance between them, a【C15】______ discovered by Newton. The amount of mass and the distance between objects also【C16】______ how fast the objects travel as they orbit each other. The force of its gravity keeps Earth and the other planets【C17】______ orbit around it. The orbital speeds of the planets【C18】______ with increasing distance from the sun. The planet nearest to the sun travels at an orbital speed of 48 kilometers per second【C19】______ the distant planet Pluto orbits【C20】______ speed of 7 kilometers per second
【C11】
选项 A、appears
B、used
C、resorts
D、is sure
答案A
解析 表不确定。
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