A source of sound emits sound waves at frequency \(f_0\). It is moving towards an observer with fixed speed \(v_s\left(v_s < v\right.\), where \(v\) is the speed of sound in air.) If the observers were to move towards the source with speed \(v_0\), one of the following two graphs \((A\) and \(B)\) will give the correct variation of the frequency \(f\) heard by the observer as \(v_0\) is changed.


The variation of \(f\) with \(v_0\) is given correctly by

1. A \(\operatorname{graph} A\) with slope \(=\dfrac{f_0}{\left(v+v_s\right)}\)
2. B graph \(B\) with slope \(=\dfrac{f_0}{\left(v-v_s\right)}\)
3. C graph \(A\) with slope \(=\dfrac{f_0}{\left(v-v_s\right)}\)
4. D graph \(B\) with slope \(=\dfrac{f_0}{\left(v+v_s\right)}\)