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  3. Over the centuries, scientists have been trying hard to find satisfactory substitutes for precious stones but failed many times.

Over the centuries, scientists have been trying hard to find satisfactory substitutes for precious stones but failed many times.

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问题     Over the centuries, scientists have been trying hard to find satisfactory substitutes for precious stones but failed many times.
    Many years ago, a chemist surprised the world by saying: It should be possible to make a precious stone that not only looks like the real thing, but that is the real thing.The only difference should be that one crystal would be made by man, the other by nature.
    At first this did not seem like a particularly hard task. Scientists began to try making synthetic diamonds towards the end of the eighteenth century and discovered a key scientific fact: diamonds are a form of carbon, which is a very common element.
    Then, why not change a cheap and plentiful substance, carbon, into a rare and expensive one, diamond?This change takes place in nature, so it should be possible to make it happen in the laboratory.
    It should be possible, but for 150 years every effort failed though several people believed that they had solved the diamond riddle during this period.
    The real success came after the scientists realized that, in changing carbon into diamonds, unbelievably heavy pressure and extremely high temperature are indispensable, and carbon needs to be dissolved(溶解) in a metal that helps the reaction to take place more easily.
    Under these conditions, a number of shiny crystals were produced in the laboratories of the General Electric Company.The results of careful analysis showed that the chemist was right: these jewels of the laboratory were not like diamonds; they were diamonds.
If something is indispensable (Paragraph 6), it is _____.
选项 A、enormously helpful
B、relatively useless
C、absolutely necessary
D、reasonably valuable
答案C
解析
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