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A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of $15{\mathrm{ms}}^{-1}$. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is

(Take, velocity of sound in air=)

(a) 800 Hz

(b) 838 Hz

(c) 885 HZ

(d) 765 Hz

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