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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 frequenc

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

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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 frequency 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 carry light signals. When the light reached 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 1977. During the experiment, two glass fibers were able to carry 672 conversations at the same time. The light-wave cable, containing 144 glass fibers, has the capacity to carry 50,000 conversations at the same time.
    The light-wave communication system has two important advantages. First, the glass fiber cables are smaller and weigh less than copper cables. 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、light must first be changed into electrical signals and then into sound
C、the light signals have to be turned directly into sound messages
D、electrical signals have to be changed directly into sound messages
答案B
解析
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