A car is moving in a straight line at constant speed and passes near a stationary observer. The observer records the frequency of the sound produced by the engine. When the car is coming towards the observer, the measured frequency is $220\,\text{Hz}$. When the car is travelling away, the measured frequency is $180\,\text{Hz}$. The speed of sound in air is $340\,\text{m s}^{-1}$. What is the speed of the car?
- A$8.5\,\text{m s}^{-1}$
- B$31\,\text{m s}^{-1}$
- C$34\,\text{m s}^{-1}$
- D$38\,\text{m s}^{-1}$