A sound source emitting a wave of constant frequency \(f_0\) moves at a constant speed along a straight horizontal track. A stationary listener is located at a perpendicular distance \(d\) from the track. The source passes its point of closest approach to the listener at time \(t = t_0\). Which of the following graphs best represents the observed frequency \(f\) heard by the listener as a function of time \(t\)?