Which of the following best describes the Doppler Effect? a. The frequency of a sound appears higher as the source of the sound moves closer and the frequency of the sound appears lower as the source moves away from the observer.
b. The wavelength of a sound appears shorter as the source of the sound moves closer and the wavelengths of a sound appear longer as the source of the sound moves away from the observer.
c. A phenomenon in which sound waves will compress as the source of the sound moves towards the observer, and rarefaction as the source of the sound moves away from the observer.
d. All of the above.