An infinitely long straight non-magnetic conducting wire of radius $$a$$ carries a dc current $$I$$. The magnetic field $$B$$, at a distance $$r$$ $$(r < a)$$ from axis of the wire is

A

$$\frac{\mu_0 I}{2\pi a}$$

B

$$\frac{\mu_0 I r}{2\pi a^2}$$

C

$$\frac{2\mu_0 I r}{\pi a^2}$$

D

$$\frac{\mu_0 I r^2}{2\pi a^3}$$