42. An ambulance siren emits a frequency of 620 Hz and the speed of sound is 340 m/s. Which statement below is correct?
(A) A stationary observer always hears a frequency of 620 Hz.
(B) When the ambulance moves towards a stationary observer at 30 m/s, the observer hears a frequency of 310 Hz.
(C) When the ambulance moves away from a stationary observer at 30 m/s, the observer hears a frequency of 680 Hz.
(D) When the ambulance moves towards a stationary observer at 30 m/s, the observer hears a frequency of 680 Hz.
(E) When the ambulance moves away from a stationary observer at 30 m/s, the observer hears a frequency of 310 Hz.

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