A train moves along a straight track with constant speed of $30\,\text{m s}^{-1}$. Its horn emits a continuous sound at frequency $2400\,\text{Hz}$. An observer is at rest beside the railway line. As the train comes towards the observer, passes him, and then recedes. The speed of sound is $340\,\text{m s}^{-1}$. What is the greatest difference between the frequencies of the sound heard by the observer?
- A$190\,\text{Hz}$
- B$230\,\text{Hz}$
- C$430\,\text{Hz}$
- D$460\,\text{Hz}$