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 \(v_0\), one of the following two graphs (\(A\) and \(B\)) will given 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 graph \(A\) with slope \( = \,\frac{{{f_0}}}{{\left( {v + {v_s}} \right)}}\)
2. B graph \(B\) with slope \( = \,\frac{{{f_0}}}{{\left( {v - {v_s}} \right)}}\)
3. C graph \(A\) with slope \( = \,\frac{{{f_0}}}{{\left( {v - {v_s}} \right)}}\)
4. D graph \(B\) with slope \( = \,\frac{{{f_0}}}{{\left( {v + {v_s}} \right)}}\)