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 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 \( v_0 \) changes.