A circular metal plate of radius R is rotating with a uniform $\mathrm{\omega }$ angular velocity  with its plane perpendicular to a uniform magnetic field B. Then the emf developed between the centre and the rim of the plate is

• A

  ${\mathrm{\pi \omega BR}}^2$

• B

  ${\mathrm{\omega BR}}^2$

• C

  ${\mathrm{\pi \omega BR}}^2/2$

• D

  ${\mathrm{\omega BR}}^2/2$