A loudspeaker produces sound with frequency $f_s$. The loudspeaker is fitted to a car that travels straight towards a stationary observer and its speed is increasing. Which statement gives the frequency of the sound heard by the observer?
- Aa frequency greater than $f_s$ and increasing
- Ba frequency greater than $f_s$ but decreasing
- Ca frequency less than $f_s$ and decreasing
- Da frequency less than $f_s$ but increasing