An object is moving at a constant speed v₀ towards a source that is at rest and that is emitting sound waves of frequency f₀. The frequency of the echo that returns to the source after being reflected from the object is given by:
(a) f_echo = f₀ v/(v-v₀)
(b) f_echo = f₀ (v-v₀)/(v+v₀)
(c) f_echo = f₀ (v+v₀)/(v-v₀)
(d) f_echo = f₀ (v+v₀)/v