Which of the following accurately describes the Doppler effect? a. As an object emitting a sound approaches an observer, the sound waves expand, so that the observer hears a higher frequency. b. As an object emitting a sound approaches an observer, the sound waves expand, which reduces their energy and dampens the noise. c. As an object emitting a sound approaches an observer, the sound waves become compressed, so that the observer hears a higher frequency. d. As an object emitting a sound approaches an observer, the sound waves become compressed, so that the observer hears a lower frequency