2. 外语
  3. A scientist once said that studying the stars from the earth is【C1】______ looking up at the sky from the bottom of a switching p

A scientist once said that studying the stars from the earth is【C1】______ looking up at the sky from the bottom of a switching p

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问题     A scientist once said that studying the stars from the earth is【C1】______ looking up at the sky from the bottom of a switching pool. Even on the clearest night the earth’s dense atmosphere【C2】______ 30 percent of the light from any star. Air turbulence may cause stars to twinkle.【C3】______  a telescope the stars seem to jump around dizzily.
    Because the moon has no atmosphere, astronomers are looking forward to【C4】______ an observatory there. On the moon, there will be no clouds to【C5】______ the view. The sky will appear dark【C6】______ , so astronomers can work twenty-four hours a day. Because there is no air turbulence, the stars will appear【C7】______ through a telescope.
   【C8】______ the moon is relatively near the earth, moon-based astronomers will see the stars in the familiar patterns known on【C9】______ . They will not need new star maps of the sky. Stars【C10】______ the moon will appear to move very slowly. Because the earth【C11】______  on its axis once every twenty-four hours earth based astronomers see the stars hurry  【C12】______  the sky each night. The moon rotates on its axis only once a month. To astronomers on the moon, the stars will appear to stand【C13】______ still. Moon-based astronomers will have much more time【C14】______  their telescopes and take time-exposure photographs.
    On the earth, mechanical radio noises【C15】______ the airwaves and interfere with the【C16】______ of radio signals from certain stars. A radio telescope【C17】______ on the far side of the moon will be permanently protected from such 【C18】______ .
    It won’t be easy to put telescopes on the moon. However, astronomers are【C19】______ the job can be done. They are convinced that the moon is an【C20】______ place to set up an observatory.
【C6】
选项 A、at times
B、at one time
C、at odd times
D、at all times
答案D
解析 as times意为“有时，偶尔”；at one time意为“曾经，有一回”；at odd times意为“偶尔，在空闲时候”；at all times意为“总是，始终”。句中can work 24 hours a day证明答案是D，因为天空总是漆黑一片，所以，科学家能够一天工作24小时。
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