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_oNI)/ b`
(b) `(2mu_o NI)/ a`
(c) `(''_oNI)/(2(b-a))ln(b/a)`
(d) `o/(2NI(b-a))ln(ab)`