The source producing sound and an observer are both moving along the direction of propagation of sound waves. If the respective velocities of sound,source,and an observer are $v$,$v_s$,and $v_o$,then the apparent frequency heard by the observer will be ($n =$ frequency of sound).

• A
  $\frac{n(v + v_o)}{v - v_o}$
• B
  $\frac{n(v - v_o)}{v - v_s}$
• C
  $\frac{n(v - v_o)}{v + v_s}$
• D
  $\frac{n(v + v_o)}{v + v_s}$