Astronauts in deep space use radio devices to communicate with each other. The radio device converts the sound produced by the first astronaut into radio signals that reach to the radio device of the second astronaut. There, this radio signal converts back into sound so that the second astronaut can hear the voice of the first. In outer space, astronauts use radio devices instead of direct sound to communicate because

Options

A. sound cannot travel in vacuum
B. it is very difficult to speak in vacuum
C. radio devices are very cheap and easily available
D. vocal cords do not vibrate to produce sound in outer space.