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  3. A computer is a machine designed to perform work mathematically and to store and select information that has been fed into it. I

A computer is a machine designed to perform work mathematically and to store and select information that has been fed into it. I

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问题     A computer is a machine designed to perform work mathematically and to store and select information that has been fed into it. It is run by either mechanical or electronic means. These machines can do a great deal of complicated work in a very short time. A large computer, for example, can add or subtract nine thousand times a second, multiply a thousand times a second, or divide five hundred times a second. Its percentage of error is about one in a billion digits . It has been estimated that human beings making calculations average about one mistake per two hundred digits.
    The heart of an electronic computer lies in its vacuum tubes, or transistors. Its electronic circuits work a thousand times faster than the nerve cells in the human brain. A problem that might take a human being two years to solve can be solved by a computer in one minute, but in order to work properly, a computer must be given instructions—it must be programmed.
    Computers can be designed for many specialized purposes—they can be used to prepare payrolls, guide airplane flights, direct traffic, even play chess. Computers play an essential role in modern automation in many plants and factories throughout the world.  
A computer is a machine designed to______.
选项 A、perform work mathematically
B、perform complicated calculations
C、store and select information
D、all of the above
答案D
解析 由文章第一段第一句“A computer is amachine designed to perform work mathematicallyand to store and select information that has beenfed into it．”及后面的“These machines can do agreat deal of complicated work in a very shorttime．”可知D项正确。
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