A conducting circular loop is placed in a uniform magnetic field of induction B tesla with its plane normal to the field. Now, radius of the loop starts shrinking at the rate (dr/dt). Then the induced e.m.f at the instant when the radius is r, is

• A

  $\mathrm{\pi rB}\left(\frac{\mathrm{dr}}{\mathrm{dt}}\right)$

• B

  $2\mathrm{\pi rB}\left(\frac{\mathrm{dr}}{\mathrm{dt}}\right)$

• C

  ${\mathrm{\pi r}}^2\mathrm{B}\left(\frac{\mathrm{dr}}{\mathrm{dt}}\right)$

• D

  $\frac{{\mathrm{\pi Br}}^2}{2}\left(\frac{\mathrm{dr}}{\mathrm{dt}}\right)$