A uniform magnetic field exists in region which form an equilateral triangle of side a. The magnetic field is perpendicular to the plane of the triangle. A charge q enters into this magnetic field perpendicularly with speed v along perpendicular bisector of one side and comes out along perpendicular bisector of other side. The magnetic induction in the triangle is -
(A) mv/qa
(B) 2mv/qa
(C) mv/2qa
(D) mv/4qa
