$A$ wire bent in the shape of a regular $n$-polygonal loop carries a steady current $I$. Let $l$ be the perpendicular distance of a given segment and $R$ be the distance of a vertex from the centre of the loop. The magnitude of the magnetic field at the centre of the loop is given by

• A
  $\frac{n \mu_0 I}{2 \pi l} \sin (\pi / n)$
• B
  $\frac{n \mu_0 I}{2 \pi R} \sin (\pi / n)$
• C
  $\frac{n \mu_0 I}{2 \pi l} \cos (\pi / n)$
• D
  $\frac{n \mu_0 I}{2 \pi R} \cos (\pi / n)$