An ambulance emits a warning signal at a frequency of $600\,\text{Hz}$. The speed of sound is $330\,\text{m s}^{-1}$. The ambulance moves at a constant velocity of $25\,\text{m s}^{-1}$ towards an observer. After it passes, it continues travelling away from the observer with the same velocity. What net change in the observed frequency occurs from the time when the ambulance is far behind the observer to the time when it is far in front of the observer?
- A$49\,\text{Hz}$
- B$84\,\text{Hz}$
- C$91\,\text{Hz}$
- D$98\,\text{Hz}$