The diagram shows a train travelling in a straight line at constant speed v, as it approaches the platform of a station.

The whistle of the engine is emitting a sound of constant frequency. Which of the following is not true for the sound of the whistle heard by an observer on the platform?

A. A sudden change in frequency of the sound as the train passes the observer.
B. A sound of constant frequency as the train approaches the observer.
C. A sound of increasing frequency as the train approaches the observer and of decreasing frequency after the train has passed the observer.
D. A sound of constant frequency after the train has passed the observer.

C