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=B_0t`. Neglecting magnetism induced due to rotational motion of ring.
The magnitude of induced emf of the closed surface of ring will be
A. `pi a^(2) B_(0)`
B. `2a^(2)B_(0)`
C. zero
D. `(1)/(2) pi a^(2) B_(0)`
