A police car is fitted with a two-tone siren that produces sounds with frequencies of $700\,\text{Hz}$ and $1000\,\text{Hz}$. The car is moving at $40.0\,\text{m s}^{-1}$ towards a stationary observer. The speed of sound in air is $340\,\text{m s}^{-1}$. What is the difference between the two frequencies heard by the observer?
- A$268\,\text{Hz}$
- B$300\,\text{Hz}$
- C$335\,\text{Hz}$
- D$340\,\text{Hz}$