Answer :
That change in pitch can be described by the math
of the Doppler effect. But that doesn't 'explain' it.
The correct answer is:
Doppler effect
Explanation:
The Doppler effect is the variation in wavelength or wavelength of a wave in relative to an observer who is moving to correspond to the wave source. It is described after the Austrian physicist Christian Doppler, who explained the phenomenon in 1842.