A train travelling in a straight line emits a sound of constant frequency f . An observer at rest v
A train travelling in a straight line emits a sound of constant frequency f . An observer at rest very close to the path of the train detects a sound of continuously decreasing frequency. The train is
A. approaching the observer at constant speed.
B. approaching the observer at increasing speed.
C. moving away from the observer at constant speed.
D. moving away from the observer at increasing speed.