A man beside a train that is at rest hears a sound of frequency $400\,\text{Hz}$ coming from the train’s horn. The train then travels directly away from the man and blows its horn when it is moving at a speed of $50\,\text{m s}^{-1}$. The speed of sound is $340\,\text{m s}^{-1}$. What is the change in frequency of the sound heard by the man between the two cases?
- A$51\,\text{Hz}$
- B$69\,\text{Hz}$
- C$349\,\text{Hz}$
- D$469\,\text{Hz}$