When a source of sound is moving towards a stationary observer, the Doppler effect demonstrates that the observer hears an apparent change in frequency in the sound. Which of the following statements best explains why the observer hears an apparent change in frequency?
a) The wavelength of the sound changes as the source moves towards the observer.
b) The frequency of the source changes as it moves towards the observer.
c) The speed of the sound changes as the source moves towards the observer.
d) The amplitude of the sound changes as the source moves towards the observer.