The siren that sails past you drops its pitch the instant it passes — that is the Doppler effect, and the sanity rail you must never let go of: approach raises pitch, retreat lowers it. Any answer that disagrees is wrong before the arithmetic starts.

Two machines, depending on who moves. A moving SOURCE squeezes its own wavefronts: $f' = \dfrac{f v}{v - v_s}$ — read aloud: f-prime equals f v over v minus v-s ($f'$ is said f-prime, the pitch actually heard). A moving LISTENER meets wavefronts more often: $f' = \dfrac{f (v + v_o)}{v}$. Approach makes both fractions top-heavy — the rail, built into the algebra.

And when two nearly equal tones sound together, they drift in and out of step and the loudness pulses: beats, at $f_{\text{beat}} = f_1 - f_2$ with $f_1$ the higher. A violinist tuning to a fork listens for the beats to slow to nothing — silence between two pitches is what "in tune" means.