A Stationary Emitter Generates a Sound at Some Particular Frequency

Multiple Choice

A stationary emitter generates a sound at some particular frequency. The frequency detected by a receiver moving away from the emitter at speed v_r (< the speed of sound in still air) is roughly the same as if the

A) emitter were to move at the same speed v_r away from a stationary receiver.
B) emitter were to move at the same speed v_r toward a stationary receiver.
C) receiver were to move at the same speed v_r toward a stationary receiver.
D) none of the above.