$A$ source of sound emits sound waves at frequency $f_0$. It is moving towards an observer with fixed speed $v_s$ ($v_s < v$,where $v$ is the speed of sound in air). If the observer were to move towards the source with speed $v_0$,the variation of the frequency $f$ heard by the observer as $v_0$ is changed is given by one of the following two graphs ($A$ and $B$). The variation of $f$ with $v_0$ is given correctly by:

• A
  graph $A$ with slope $= \frac{f_0}{v - v_s}$
• B
  graph $B$ with slope $= \frac{f_0}{v - v_s}$
• C
  graph $A$ with slope $= \frac{f_0}{v - v_s}$
• D
  graph $B$ with slope $= \frac{f_0}{v + v_s}$