A uniform magnetic field exists in a region which forms an equilateral triangle of side $a$. The field is perpendicular to the plane of the triangle. A charge $q$ enters the triangle with speed $v$ along the $\perp$ bisector of one of the sides and comes out along the $\perp$ bisector of the other side. Intensity of the magnetic field is

Question

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