Figure shows a ring of radius ‘a’ at t = 0, present in a uniform magnetic field B. The magnetic field is present everywhere parallel to the axis $\mathrm{yy}\text{'}$  . If ring starts rotating with an angular speed   $\mathrm{\omega }$ about axis   $\mathrm{yy}\text{'}$, the induced emf in ring is 
 

• A

  $\mathrm{\pi }{\mathrm{a}}^2\mathrm{B}\mathrm{\omega }\sin \mathrm{\omega t}$

• B

  $\mathrm{\pi }{\mathrm{a}}^2\mathrm{B}\mathrm{\omega }\text{ cos }\mathrm{\omega t}$

• C

  $\mathrm{\pi }{\mathrm{a}}^2\text{B }\mathrm{\omega }$

• D

  Zero