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The correct answer is:
d. all of the above.
Let's break it down:
1. AM radio waves have longer wavelengths than FM radio waves. Wavelength is the distance between successive crests of a wave. In the case of radio waves, AM waves typically have longer wavelengths than FM waves. This difference in wavelength affects how these waves travel and interact with obstacles in the environment.
2. AM radio waves are encoded with signals by changing their amplitude. Amplitude modulation (AM) is a method used to encode information onto a carrier wave by varying the amplitude of the wave. This modulation technique allows audio signals to be transmitted over the radio waves, where the variations in amplitude represent the variations in the sound signal.
3. AM radio waves can reflect off the ionosphere. The ionosphere is a layer of the Earth's atmosphere that contains ions and free electrons, allowing it to reflect certain radio waves. AM radio waves, being lower in frequency than FM waves, can bounce off the ionosphere and travel long distances by multiple reflections between the Earth's surface and the ionosphere.
Therefore, the statement "d. all of the above" is correct as it encompasses all the characteristics and behaviors associated with AM radio waves.
