Choose the correct word from the brackets. 1) We can only (communicate, communicated, communicating) information by radio waves by changing the wave in some way. This change is (know, knew, known) as modulation. The simplest form of modulation (are, is, were) to turn the wave on and off. This method was (use, used, using) in the early days of radio for telegraphic signals. The wave was (stops, stopped, stopping) and started to represent the dots and dashes of the Morse code by means of a telegraph key. 2) Speech and music (produce, producing, produces) audio frequencies which cannot be (transmitted, transmit, transmitting) directly. But they can be used to modulate radio waves. The modulated wave is then transmitted. When it is (receive, received, receives), the wave is modulated and the original audio frequency signal is recovered. The radio wave (act, acting, acts) only to carry the audio frequency signal and is (call, called, calling) the carrier wave. The audio frequency signal is termed the modulating signal. 3) A wave (have, has, had) three quantities: amplitude, frequency and phase. Any of these quantities can be modulated. The two commonest methods of modulation (is,was, are) and amplitude modulation and frequency modulation. No information can be communicated by radio waves (whether, without, with) modulating them. In amplitude modulation the amplitude of the carrier wave is (charged, changed, created) according to the amplitude of the modulated (single, signal, simple). In frequency modulation the amplitude of the carrier wave is kept constant.Frequency modulation has two advantages over AM. It is less (effected, affected, reflected) by interference and it reproduces music more faithfully. A VHF receiver is more complicated (then, than, when) one for amplitude modulation, but the reception is much better.