A thin non-conducting ring of mass m, radius a carrying a charge q can rotate freely about its own axis which is vertical. At the initial moment, the ring was at rest in horizontal position and no magnetic field was present. At instant t=0, a uniform magnetic field is switched on which is vertically downward and increases with time according to the law B=B0t. Neglecting magnetism induced due to rotational motion of ring.
Angular acceleration of ring is
(a) 2mqB0
(c) mqB0
(b) 4mqB0
(d) m2qB0