$A$ charge $q$ $C$ moves in a circle at $n$ revolutions per second and the radius of the circle is $r$ $m$. The magnetic field at the centre of the circle is:

• A
  $\frac{2\pi q}{nr} \times 10^{-7} \text{ N/A} \cdot \text{m}$
• B
  $\frac{2\pi q}{r} \times 10^{-7} \text{ N/A} \cdot \text{m}$
• C
  $\frac{2\pi nq}{r} \times 10^{-7} \text{ N/A} \cdot \text{m}$
• D
  $\frac{2\pi q}{r} \text{ N/A} \cdot \text{m}$