A coil having N turns is wound tightly in the form of a spiral with inner and outer radii `a` and `b` respectively. When a current `I` passes through the coil, the magnetic field at the centre is

Options

A. `(mu_0NI)/b`
B. `(2mu_0NI)/a`
C. `(mu_0NI)/(2(b-a)) l n fracb a`
D. `(mu_0I^N)/(2(b-a)) l n fracb a`.