Two concentric circular coils having radii $r_{1}$ and $r_{2}$ $(r_{2} \ll r_{1})$ are placed coaxially with centers coinciding. The mutual inductance of the arrangement is (Both coils have a single turn) ($\mu_{0} =$ permeability of free space).

• A
  $\frac{\mu_{0} \pi r_{2}^{2}}{r_{1}}$
• B
  $\frac{\mu_{0} \pi r_{1}^{2}}{r_{2}}$
• C
  $\frac{\mu_{0} \pi r_{1}^{2}}{2 r_{2}}$
• D
  $\frac{\mu_{0} \pi r_{2}^{2}}{2 r_{1}}$