The driver of a car travelling with a speed $V_1 \ m/s$ towards a wall sounds a siren of frequency $n \ Hz$. If the velocity of sound in air is $V \ m/s$,then the frequency of sound reflected from the wall and as heard by the driver,in $Hz$,is

• A
  $\left(\frac{V+V_1}{V-V_1}\right) n$
• B
  $\left(\frac{V-V_1}{V+V_1}\right) n$
• C
  $\left(\frac{V_1-V}{V_1+V}\right) n$
• D
  $\left(\frac{V_1}{V_1-V}\right) n$