An observer is standing 500 m away from a vertical hill, a police van sounding a siren of frequency 1000 Hz moves towards the hill with a uniform speed. If the frequency of the sound heard directly from the siren is 970 Hz. the frequency of the sound heard after reflection from the hill (in Hz) is about, (velocity of sound =$330\;ms^{-1}$):

Question

\[\begin {array} {1 1} (a)\;1042 & \quad (b)\;1032 \\ (c)\;1022 & \quad (d)\;1012 \end {array}\]