A train driver blows the train's horn as it moves away from a station. Which of the following statements describes how the sound of the horn heard by an observer standing at the station platform differs from the sound heard by the train driver?
The observer hears the sound as having a lower frequency.
The observer hears the sound as having a greater velocity.
The observer hears the sound as having a greater amplitude.
The observer hears the sound as having a shorter wavelength.