A bicyclist rings a bell that has a rest frequency of 700 Hz while riding past a person standing on a sidewalk. The pedestrian notices that the pitch she hears is higher than 700 Hz until the instant the bicycle is directly alongside her, after which the pitch immediately becomes lower than 700 Hz. Which of the following best explains this sudden change in the observed frequency?
(a) The sound amplitude decreases after the bicycle passes; because lower-intensity sound is perceived as lower in frequency, the pitch seems to drop.
(b) When the bicycle reaches the closest point, the component of its velocity toward the pedestrian instantly changes sign, so the Doppler shift switches from an upward to a downward shift.
(c) Air dragged along by the moving bicycle changes the speed of sound behind it; once the bicycle passes, the greater speed of sound stretches the wavelength and lowers the frequency you hear.
(d) After the bicycle passes, air resistance slows the bell slightly, reducing the actual frequency it emits and therefore the pitch you hear.