The frequency of the whistle of a train moving with a constant speed is observed by a stationary detector on the platform to be $\nu_1$. The frequency of the same whistle is detected to be $\nu_2$ inside another train moving on a parallel track, at a speed $v$ towards the first train. If the speed of sound in air is taken to be $v_{\text {sound }}$, what is the ratio $v / v_{\text {sound }}$ ?

• Correct answer
  A

  $\frac{\nu_2-\nu_1}{\nu_1}$
• B

  $\frac{\nu_2}{\nu_1}$
• C

  $\frac{\nu_2-\nu_1}{\nu_2}$
• D

  $\frac{\nu_1}{\nu_2}$

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