A source of sound of frequency 640 Hz is moving at a velocity of $\large\frac{100}{3}$$\;m/s$ along a road, and is at an instant 30 m away from a point A on the road (as shown in figure). A person standing at $O$, $40 \;m$ away from the road hears sound of apparent frequency $v^1$. The value of $v^1$ is (velocity of sound= 340 \;m/s)

Question

$$(a)\;620\;Hz \quad (b)\;680\;Hz \quad (c)\;720\;Hz \quad (d)\;840\;Hz$$