An observer is standing on a railway platform. A train moves past the observer at a steady speed while its whistle emits sound with constant frequency $f$. What is heard by the observer?
- Asound of a decreasing frequency as the train approaches and of an increasing frequency as it moves away
- Bsound of a higher frequency than $f$ as the train approaches and of a lower frequency than $f$ as the train moves away
- Csound of a lower frequency than $f$ as the train approaches and of a higher frequency than $f$ as the train moves away
- Dsound of an increasing frequency as the train approaches and as it moves away