单项选择题

If you have a Phobia,______.

A. you must take swimming lessons in a safe pool

B. you’d better practise looking down from middle heights first

C. you may attend some parties

D. it will be as troublesome as an infectious disease

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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 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

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.