A train whistle produces sound with frequency $500\,\text{Hz}$ while the train travels at a speed of $20\,\text{m s}^{-1}$ along a straight track. The train is moving straight towards a stationary observer beside the track, and then it goes past the observer. The speed of sound in air is $330\,\text{m s}^{-1}$. What is the difference between the frequencies of the sound heard by the observer before the train passes and after it has passed the observer?
- A$29\,\text{Hz}$
- B$32\,\text{Hz}$
- C$40\,\text{Hz}$
- D$61\,\text{Hz}$