A coil having N turns is wound tightly in the form of a spiral with inner and outer radii a and b respectively. A current I passes through the coil. The magnetic moment of the spriral is
A. `(mu_(0)NI)/(b)`
B. `(2mu_(0)NI)/(a)`
C. `(mu_(0)NI)/(2(b-a))"In" (b)/(a)`
D. `(2mu_(0)I^(N))/(2(b-a))"In"(b)/(a)`
