The driver of a car traveling with speed $30 \mathrm{~m} / \mathrm{s}$ towards a hill sound a horn of frequency $600 \mathrm{~Hz}$. If the velocity of sound in air is $330 \mathrm{~m} / \mathrm{s}$, the frequency of reflected sound as heard by the driver is
(A) $720 \mathrm{~Hz}$
(B) $555.5 \mathrm{~Hz}$
(C) $550 \mathrm{~Hz}$
(D) $500 \mathrm{~Hz}$