A person carrying a whistle entitling continuously a note of 272 Hz is running towards a reflecting surface with a speed of $18 km h^{-1}$. The speed of sound in air is $345 ms^{-1}$ The number of beats heard by him is

Question

$\begin{array}{1 1} 4 \\ 6 \\ 8 \\ 3 \end{array} $