An electron beam in a TV picture is accelerated through a potential difference of V volts. It passes through a region of transverse magnetic induction field (B) and follows a circular orbit of radius 'r'. The induced magnetic field (B) is

Question

$\begin {array} {1 1} (1)\;\sqrt{\large\frac{2mV}{er^2}} & \quad (2)\;\sqrt{\large\frac{2mV}{r}} \\ (3)\;\sqrt{\large\frac{2mVr}{e}} & \quad (4)\;\large\frac{2mV}{{\sqrt{er}}} \end {array}$