The magnitude of magnetic induction at a point on the axis at a large distance $(r)$ from the centre of a circular coil of $n$ turns and area $A$ carrying current $I$ is given by:

• A
  $B_{\text{axis}} = \frac{\mu_0}{4 \pi} \cdot \frac{n A}{I r^3}$
• B
  $B_{\text{axis}} = \frac{\mu_0}{4 \pi} \cdot \frac{2 n I A}{r^3}$
• C
  $B_{\text{axis}} = \frac{\mu_0}{4 \pi} \cdot \frac{2 n I}{A r^3}$
• D
  $B_{\text{axis}} = \frac{\mu_0}{4 \pi} \cdot \frac{n I A}{r^3}$