A Locomotive Moving at a Constant Speed Approaches and Then

Multiple Choice

A locomotive moving at a constant speed approaches and then passes a man standing at a crossing.The frequency of the locomotive's horn as the man hears it

A) gradually increases and then decreases.
B) gradually decreases and then increases.
C) abruptly changes as the locomotive passes.
D) does not change at all.
E) produces beats due to the Doppler effect.