The driver of bus approaching a big wall notices that the frequency of his bus's horn changes from $420 \mathrm{Hz}$ to $490 \mathrm{Hz}$ when he hears it after it gets reflected from the wall. Find the speed of the bus if speed of the sound is $330 {\mathrm{ms}}^{–1}$:

• Correct answer
  A

  $91 {\mathrm{kmh}}^{-1}$
• B

  $81 {\mathrm{kmh}}^{-1}$
• C

  $61 {\mathrm{kmh}}^{-1}$
• D

  $71 {\mathrm{kmh}}^{-1}$

Hence, the correct answer is option: 'A'.