An ambulance siren produces a sound of single frequency $f$. The ambulance moves towards a stationary observer, passes very close by, and then moves away from the observer. Which statement gives the frequency of the sound detected by the observer as the ambulance goes past?
- Aequal to $f$ and decreasing
- Bequal to $f$ and increasing
- Cgreater than $f$ and constant
- Dless than $f$ and constant