单选题 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.

单选题 In the 2^nd paragraph, what is the chemist trying to say?
A．it is a real thing that man was able to make a precious stone as nature can do.
B．man should be able to make a precious stone that is exactly the same as a natural one.
C．there is only a little difference between a man-made crystal and a nature-made one.
D．scientists expect a small difference between a man-made crystal and a nature-made one.

单选题 After the key scientific fact was discovered, the scientists ______.
A．felt it even harder to succeed in making synthetic diamonds
B．soon succeeded in changing cheap carbon into expensive diamonds
C．felt it more hopeful to succeed in making synthetic diamonds
D．found that carbon is very common in nature and so are diamonds

单选题 According to the passage, the real success probably came ______.
A．in the 19^th century B．around 1900
C．150 years ago D．around 1950s

单选题 If something is indispensable (Paragraph 6), it is ______.
A．enormously helpful B．relatively useless
C．absolutely necessary D．reasonably valuable

单选题 Which of the following is true according to the passage?
A．since carbon is a plentiful substance we can easily make synthetic diamonds.
B．something taking place in nature may also take place in the laboratory.
C．since diamonds are a form of carbon, diamonds can be made only by nature.
D．synthetic diamonds made in the laboratory are not like natural diamonds.