Charge $$q$$ is uniformly distributed over the surface of a thin non-conducting annular disc of inner radius $$R_1$$ and outer radius $$R_2$$. The disc is made to rotate with constant frequency $$f$$, about an axis passing through the center of the annular disc and perpendicular to its plane. The magnetic moment of the disc is

A

$$\pi fq\frac{R_2^2 + R_1^2}{2}$$

B

$$\pi fq\frac{R_2^2 - R_1^2}{2}$$

C

$$\pi fq\frac{R_2^2 - R_1^2}{4}$$

D

$$2\pi fq\left(R_2^2 - R_1^2\right)$$