A car moving along a straight path at a speed of $30\,\text{m s}^{-1}$ passes a stationary observer while its horn is sounding. The actual frequency of the sound from the horn is $400\,\text{Hz}$. The speed of sound in air is $336\,\text{m s}^{-1}$. What is the change in the frequency of the sound heard by the observer as the car goes past?
- A$39\,\text{Hz}$
- B$66\,\text{Hz}$
- C$72\,\text{Hz}$
- D$78\,\text{Hz}$