A coil having n turns is wound tightly in the form of a spiral with inner and outer radii 'a' and 'b' respectively. Find the magnetic field at the center when a current I passes through the coil:
A) (μ₀nI)/(2πa)
B) (μ₀nI)/(2πb)
C) (μ₀nI)/(π(a² + b²))
D) (μ₀nI)/(π(a + b))