A 500 Hz whistle is moved toward a listener at a speed of 10 m/s. At the same time, the listener moves at a speed of 20 m/s in a direction away fromm the whistle. What is the apparent frequency heard by the listener? (the speed of sound is 340 m/s)
A) 473 hz
B) 376 hz
C) 485 hz
D) 533 hz
E) 547hz