单项选择题

Before you can hear a message on the telephone using the new system, ______.

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 messages

C. light signals must first be changed into electrical signals and then into sound

D. either the light or the electrical signals have to be changed into sound message
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单项选择题
According to the passage the new telephone communication system, ______.
A. had been used within limited scope before 1977.
B. was first put into actual use in 1977.
C. had been repeatedly experimented on before 1977.
D. was put on experiment in 1977.
单项选择题
Which of the following statements best expresses the main idea of the passage
A. Glass is very useful because it has many unusual qualities.

B. Light signals have changed the use of glass in industry.

C. Glass fibres have reduced the cost of telephone communication.

D. The use of glass fibres to carry telephone message is an interesting new development.
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