单项选择题

According to the passage the new telephone communication system （）

A．has been put into actual use
B．was first put into actual use in 1977
C．had been repeatedly experimented on before 1977
D．was put on experiment in 1977

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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
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

单项选择题

55（）

A．what
B．why
C．when
D．how