$A$ source of sound is moving towards a stationary observer with velocity $V_s$ and then moves away with velocity $V_s$. Assume that the medium through which the sound waves travel is at rest. If $V$ is the velocity of sound and $n$ is the frequency emitted by the source, then the difference between the apparent frequencies heard by the observer is:

• A
  $\frac{2nV V_s}{V^2 - V_s^2}$
• B
  $\frac{nV V_s}{V^2 - V_s^2}$
• C
  $\frac{nV V_s}{V_s^2 + V^2}$
• D
  $\frac{2nV V_s}{V_s^2 + V^2}$