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 a, and if the circle is rotated about its axis at a rate of n rotations per second, the magnetic induction at a point on the axis at a large distance y from the centre

• A. \[{{\mu }_{0}}\pi {{r}^{3}}n\frac{\sigma }{{{y}^{3}}}\]
• B. \[2{{\mu }_{0}}\pi {{r}^{3}}n\frac{\sigma }{{{y}^{3}}}\]
• C. \[\left( \frac{{{\mu }_{0}}}{4\pi } \right){{r}^{3}}n\frac{\sigma }{{{y}^{3}}}\]
• D. \[\left( \frac{{{\mu }_{0}}}{2\pi } \right){{r}^{3}}n\frac{\sigma }{{{y}^{3}}}\]