Two sources A and B are sending notes of frequency $680\;Hz.$ A listener moves from A to B with a constant velocity 'u'. If the speed of sound in air is $340 ms^{-1}$, What must be the value of $'u'$ so that he hears 10 beats per second?

Question

$$\begin {array} {1 1} (1)\;2.0\;m-s^{-1} & \quad (2)\;2.5 m-s^{-1} \\ (3)\;3.0 m-s^{-1} & \quad (4)\;3.5 m-s^{-1} \end {array}$$