A stationary ambulance siren emits a steady tone of 800 Hz. A pedestrian standing on the sidewalk first sees the ambulance driving directly toward them and then watches it continue straight past and away. Which statement best describes the frequency the pedestrian hears during this interval?
(a) It remains exactly 800 Hz the entire time because the siren’s frequency does not change.
(b) It is less than 800 Hz while the ambulance approaches and greater than 800 Hz after it passes.
(c) It is greater than 800 Hz while the ambulance approaches and less than 800 Hz after it passes.
(d) It is greater than 800 Hz both while the ambulance approaches and while it recedes because the siren is always moving.