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
    $\frac{\mu_0 \pi r_2}{r_1}$

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