$A$ thin rod is bent in the shape of a small circle of radius $r$. If the charge per unit length of the rod is $\sigma$,and if the circle is rotated about its axis at the rate of $n$ rotations per second,the magnetic induction at a point on the axis at a large distance $y$ from the centre is

• A
  $\frac{\mu_0 \pi r^3 n \sigma}{y^3}$
• B
  $\frac{2 \mu_0 \pi r^3 n \sigma}{y^3}$
• C
  $\left( \frac{\mu_0}{4 \pi} \right) \frac{r^3 n \sigma}{y^3}$
• D
  $\left( \frac{\mu_0}{2 \pi} \right) \frac{r^3 n \sigma}{y^3}$