55. Two ambulances have identical sirens that would make sound waves of frequency700 Hz. A stationary observer is in the straight line and in between these twoambulances, while one ambulance is parked and the other is moving away theobserver. The observer notices that the combined sound wave exhibits a beatfrequency of 20 Hz. What is the speed of the ambulance moving away from theobserver? Take the speed of sound to be 340 m/s, and there is no wind blowingduring the observation.
(A) 5 m/s
(B) 10 m/s
(C) 12 m/s
(D) 17 m/s
(E) 18 m/s