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  3. The making of glass is a very old industry--at least 4,500 years old. Glass has many extraordinary qualities and it is frequentl

The making of glass is a very old industry--at least 4,500 years old. Glass has many extraordinary qualities and it is frequentl

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问题     The making of glass is a very old industry--at least 4,500 years old. Glass has many extraordinary qualities and it is frequently being used in new ways.
    One of the most interesting new uses for glass is in telephone communication. Scientists have developed glass fibers as thin as human hair, which are designed to can-y light signals. When the light reaches the other end, it is first changed into electrical signals, which are in turn converted into sound messages.
    Called light wave communication, the new system was used successfully in an experiment in Chicago in 1997. During the experiment, two glass fibers were able to carry 672 conversations at the same time. The lightwave cable, containing 144 glass fibers, has the capacity to carry 50,000 conversations at the same time.
    The lightwave communication system has two important advantages. First, the glass fiber cables are smaller and weigh less than copper. Second, they cost less.
    Perhaps it can be said that telephone communication has entered the age of light.
According to the passage the new telephone communication system ______.
选项 A、has been put into actual use
B、was first put into actual use in 1977
C、had been repeatedly experimented on before 1977
D、was put on experiment in 1977
答案D
解析 第三段第一句话是该题答案，可以判断D正确。
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