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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.
Before you can hear a message on the telephone using the new system, ______.
选项 A、electrical signals must first be changed’ into light signals and then into sound
B、the light signals have to be changed directly into sound
C、light must first be changed into electrical signals and then into sound
D、neither the light nor the electrical signals have to be changed in any way
答案C
解析 从第二段“When the light reaches the other end，…sound message”一句中，可以判断C正确。
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