45. A long coaxial cable consists of a solid inner conductor of radius a carrying current I and a thin outer shell of radius b carrying return current I. The magnetic field at distance r (a < r < b) is ______. (Given: permeability constant μ₀)

(A) 0
(B) μ₀I/(2πa)
(C) μ₀I/(2πr)
(D) μ₀Ir/(2πab)
(E) μ₀I/(2πb)

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