How are amplitude modulation and frequency modulation techniques similar?

a. both are used by cell phones to produce images.
b. both transform electrical signals into sound waves.
c. both modify the pulse of a carrier wave.
d. both are used by radio stations to transmit sound.

The correct option is "Both are used by radio stations to transmit sound.".

Explanation:

Amplitude modulation: The modulation in which the amplitude of the carrier wave is varied with the input signal. But the frequency of the carrier wave remains constant. It is used by radio station to transmit sound. It is used to deliver video signal on television.

Frequency modulation: The modulation in which the frequency of the carrier wave is varied with the input signal. While the amplitude and the phase of the carrier wave remain constant. It is used by radio station to transmit sound. It is used to deliver audio signal on the television.

Therefore, both are used by radio stations to transmit sound.  

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