The amount of energy gained in half a revolution by an electron of mass m, velocity v and charge e in a uniform magnetic field is

Question

$\begin {array} {1 1} (a)\;\large\frac{1}{2}mV^2 & \quad (b)\;\large\frac{1}{4}mV^2 \\ (c)\;mV^2 & \quad (d)\;Zero \end {array}$