Because of the Doppler Effect, the Sound of the Engines

Because of the Doppler effect, the sound of the engines of an aircraft that flies past an observer

A) changes from a high pitch or frequency to a lower pitch as the plane passes the observer.
B) changes from a low pitch or frequency to a higher pitch as the plane passes the observer.
C) remains at the same pitch or frequency, but the intensity rises as the plane approaches and then falls as the plane moves away.
D) starts at a high pitch or frequency, drops to a low frequency when the plane is overhead, then rises again as the plane moves away.