When both source and listener are approaching each other,the observed frequency of sound is given by (where $V$ is the speed of sound,$V_L$ and $V_S$ are the velocities of the listener and source respectively,and $n_0$ is the radiated frequency):

• A
  $n=n_0\left[\frac{V+V_{L}}{V-V_{s}}\right]$
• B
  $n=n_0\left[\frac{V-V_{L}}{V+V_{s}}\right]$
• C
  $n=n_0\left[\frac{V-V_{L}}{V-V_{s}}\right]$
• D
  $n=n_0\left[\frac{V+V_{L}}{V+V_{s}}\right]$