A thin circular disk of radius R is uniformly charged with density $\sigma >0$ per unit area. The disk rotates about its axis with a uniform angular speed $\omega$. The magnetic moment of the disc is :

• A

  $2\pi R^4\sigma \omega$

• B

  $\pi R^4\sigma \omega$

• C

  $\frac{\pi R^4}{2}\sigma \omega$

• D

  $\frac{\pi R^4}{4}\sigma \omega$