A source of sound $\mathrm{S}$ is moving with a velocity of $50\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$ towards a stationary observer. The observer measures the frequency of the source as $1000\mathrm{ }\mathrm{H}\mathrm{z}.$ What will be the apparent frequency of the source when it is moving away from the observer after crossing him? (Take velocity of sound in air is $350\mathrm{ }\mathrm{m} {\mathrm{s}}^{-1}$ )

• Correct answer
  A

  $750\mathrm{ }\mathrm{H}\mathrm{z}$
• B

  $857\mathrm{ }\mathrm{H}\mathrm{z}$
• C

  $1143\mathrm{ }\mathrm{H}\mathrm{z}$
• D

  $807\mathrm{ }\mathrm{H}\mathrm{z}$

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