Two conducting circular loops A and B are placed in the same plane with their centers coinciding as shown in figure. The mutual inductance between them is : 

A

$$\frac{\mu_0 \pi b^2}{2a}$$

B

$$\frac{\mu_0}{2\pi} \cdot \frac{b^2}{a}$$

C

$$\frac{\mu_0}{2\pi} \cdot \frac{a^2}{b}$$

D

$$\frac{\mu_0 \pi a^2}{2b}$$