$A$ charge $q$ coulomb moves in a circle at $n$ revolutions per second and the radius of the circle is $r$ metre; then the magnetic field at the centre of the circle is
- A$\frac{2\pi qn}{r} \times 10^{-7} \text{ T}$
- B$\frac{2\pi q}{r} \times 10^{-7} \text{ T}$
- C$\frac{2\pi nq}{r} \times 10^{-7} \text{ T}$
- D$\frac{2\pi q}{r} \text{ T}$
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