55. Two ambulances have identical sirens that would make sound waves of frequency 700 Hz. A stationary observer is in the straight line and in between these two ambulances, while one ambulance is parked and the other is moving away the observer. The observer notices that the combined sound wave exhibits a beat frequency of 20 Hz. What is the speed of the ambulance moving away from the observer? Take the speed of sound to be 340 m/s, and there is no wind blowing during the observation.
(A) 5 m/s
(B) 10 m/s
(C) 12 m/s
(D) 17 m/s
(E) 18 m/s