2. 外语
  3. A scientific discovery is often made【C1】______ before someone is able to put it to use.【C2】______ , scientists discovered how to

A scientific discovery is often made【C1】______ before someone is able to put it to use.【C2】______ , scientists discovered how to

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问题      A scientific discovery is often made【C1】______ before someone is able to put it to use.【C2】______ , scientists discovered how to split atoms decades【C3】______ the invention of the atomic bomb and the use of【C4】______ power to produce electricity. But sometimes discovery and【C5】______   happen at the same time, as occurred more than thirty years ago【C6】______ the discovery of coherent light and the invention of the laser.
    The word laser【C7】______ "light amplification by stimulated emission of radiation. To many people, lasers are very mysterious, but a laser is simply a【C8】______ that produces a strong light. The light from the laser is called【C9】______ light because it is light that only moves in one direction.【C10】______ , incoherent light, like the light from the sun or light from a bulb, moves away from its source【C11】______ all directions, so its strength is very spread out. The light from a laser, moving in only one direction and【C12】______ in a narrow beam, is much stronger. Laser light is created by a【C13】______ called stimulated emission. In this process, the atoms of a certain substance, such as a crystal or a gas, are excited 【C14】______ that they produce coherent light. A person working with a laser can aim this coherent light,【C15】______ a laser beam, in any direction.
    As soon as the laser was developed, scientists began thinking of practical applications【C16】______ it. One of the earliest uses was to make extremely precise measurements of distance and speed.【C17】______ , the distance to the moon was measured to within a foot, and the speed of light was measured to 【C18】______ a thousandth of a mile per second. As time【C19】______ , many more applications for the laser were【C20】______ .
【C17】
选项 A、At the same time
B、On the contrary
C、For example
D、In a word
答案C
解析 此题考查介词的用法。通过上下文看，作者在这里举例。所以正确答案为For example。因为这里不存在比较．所以，At the same time和On the contrary不可以选用。In a word(总而言之)常用于总结，不合题意。所以．C为正确答案。
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