Two planar concentric rings of metal wire having radii $r_1$ and $r_2$ $(r_1 > r_2)$ are placed in air. The current $I$ is flowing through the coil of larger radius. The mutual inductance between the coils is given by ($\mu_0 =$ permeability of free space).

• A
  $\frac{\mu_0 \pi r_1^2}{2 r_2}$
• B
  $\frac{\mu_0 \pi r_2^2}{2 r_1}$
• C
  $\frac{\mu_0 \pi (r_1 + r_2)^2}{2 r_1}$
• D
  $\frac{\mu_0 \pi (r_1 - r_2)^2}{2 r_2}$