A siren emitting a sound of frequency $800\; Hz$ moves away from an observer towards a cliff at a speed of $15ms^{–1}$. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is : (Take velocity of sound in air = $330 \; ms^{-1}$

Question

( A ) 838 Hz
( B ) 800 Hz
( C ) 765 Hz
( D ) 885 Hz