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
One of the extraordinary qualities of glass is that it can carry ______.
A．light signals
B．sound signals
C．electric signals
D．any signals
单项选择题
According to the passage, people started to make glass ______.
A．4 500 years ago
B．nearly 4 500 years ago
C．less than 4 500 years ago
D．no less than 4 500 3,ears ago
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