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A car moves towards a hill with speed $v_c$. It blows a horn of frequency $f$ which is heared by an observer following the car with speed $v_0$. The speed of sound in air is $v$.
  • Athe wavelength of sound reaching the hill is $\frac{v}{f}$
  • Bthe wavelength of sound reaching the hill is $\frac{{v - {v_c}}}{f}$
  • Cthe beat frequency observed by the observer is $\frac{{2{v_c}(v + {v_o})\,f}}{{{v^2} - v_c^2}}$
  • Dboth $(B)$ and $(C)$
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