Doppler effect – echo from a cliff

2016 1 question

1. 1 Waves
   A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of 15 $ms^{-1}$. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is (Take, velocity of sound in air = 330 $ms^{-1}$)
   A 800 Hz B 838 Hz C 885 Hz D 765 Hz
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