$A$ charge $q$ moving in a circle of radius $r$ metre makes $n$ revolutions per second. The magnetic field at the centre of the circle is

• A
  $\frac{2 \pi q}{n r} \times 10^{-7} \text{ T}$
• B
  $\frac{2 \pi q}{r} \times 10^{-7} \text{ T}$
• C
  $\frac{2 \pi n q}{r} \times 10^{-7} \text{ T}$
• D
  $\frac{2 \pi q}{r} \text{ T}$