A sound source emits a continuous tone of constant frequency \(f_0\). At time \(t = 0\), the source begins from rest and moves along a straight line directly toward a stationary detector with a constant acceleration \(a\). The speed of sound in the air is \(v\), and the source speed remains less than \(v\) throughout the motion. Which of the following graphs best represents the frequency \(f\) detected by the stationary observer as a function of time \(t\)?