A train moves towards a stationary observer with speed $\mathrm{34 }\mathrm{m }{\mathrm{s}}^{-1}$. The train sounds a whistle and its frequency registered by the observer is $f_1$. If the train's speed is reduced to $\mathrm{17 }\mathrm{m }{\mathrm{s}}^{-1}$, the frequency registered is $f_2$. If the speed of sound is $\mathrm{340 }\mathrm{m }{\mathrm{s}}^{-1}$, then the ratio  $\frac{f_1}{f_2}$

• A

  $\frac{18}{19}$
• B

  $\frac{1}{2}$
• C

  $2$
• Correct answer
  D

  $\frac{19}{18}$

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