单项选择题

According to the passage the new telephone communication system ______.

A．has been put into actual use
B．was put into actual use in 1977
C．was put on experiment in 1977
D．had been repeatedly experimented on before 1977
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单项选择题
Which statement can be the main idea of the passage
A．Glass is very useful because it has 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 messages in an interesting new development.
单项选择题
One of the extraordinary qualities of glass is that it can carry ______.
A．light signals
B．sound signals
C．electric signals
D．any signals
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