Two concentric circular coils having radii $r_1$ and $r_2$ $(r_2 \ll r_1)$ are placed co-axially with centres coinciding. The mutual induction of the arrangement is (Both coils have single turn,$\mu_0 =$ permeability of free space).

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