单项选择题
A．saves lives of people in remote places B．enables people to live alone if they want to C．spreads both love and ill will D．replaces much written communication
A．obedient B．responsible C．cooperative D．independent
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