While Standing at a Crosswalk,you Hear a Frequency of 540

Multiple Choice

While standing at a crosswalk,you hear a frequency of 540 Hz from an approaching police car.After the police car passes,its frequency is 480 Hz.What is the speed of the police car? (speed of sound = 340 m/s)

A) 13.1 m/s
B) 17.4 m/s
C) 20.0 m/s
D) 26.2 m/s