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 [IIT-JEE (Screening) 2001]

• A.            \[\frac{{{\mu }_{0}}NI}{b}\]  
• B.            \[\frac{2{{\mu }_{0}}NI}{a}\]
• C.            \[\frac{{{\mu }_{0}}NI}{2(b-a)}\ln \frac{b}{a}\]        
• D.            \[\frac{{{\mu }_{0}}{{I}^{N}}}{2(b-a)}\ln \frac{b}{a}\]