A band playing music at a frequency f is moving towards a wall at a speed Vb. A
motorist is followed by the band with a speed V. If Vₘ is the speed of the sound, the expression for the beat frequency heard by the motorist is:
a. (V + Vₘ) / (V + Vb)f
b. (2Vb (V + Vₘ)) / (V² - Vb²)
c. (2Vₘ (V + Vb)) / (V² - Vₘ²)
d. (V + Vₘ) / (V - Vb)