$A$ wire of mass $m$ and length $l$ is bent in the form of a circular ring. The moment of inertia of the ring about its axis is:

• A
  $\left( \frac{1}{8\pi^2} \right) ml^2$
• B
  $\left( \frac{1}{2\pi^2} \right) ml^2$
• C
  $\left( \frac{1}{4\pi^2} \right) ml^2$
• D
  $ml^2$