A source of sound emits waves at frequency $f_0$ . It is moving towards an observer with fixed speed $v_s (v_s < v$, where v is the speed $v_0$, One of the following twp graphs (A and B) will give the correct variation of the frequency f heard by the observer as $v_0$ is changed . The variation f with $v_0$ is given correctly by :

Question

$\begin{array}{1 1} graph\; A \;with \;slope=\frac{f_0}{(v-v_s)} \\graph\;A\;with\;slope=\frac{f_0}{(v+0v_s)} \\ graph\;B\;with\;slope=\frac{f_0}{(v-v_s)} \\ graph\;B\;with\;slope=\frac{f_0}{(v+v_s)} \end{array}$