15. A fat wire of radius a carries a constant current I, which is uniformly distributed over its cross section. A narrow gap in the wire (of width w ≪ a) forms a parallel-plate capacitor. Inside this gap, the magnetic field at a distance s ＜ a from the central axis is (A) μ₀sI/(2πa²) (B) μ₀sI/a² (C) μ₀I/(2πs) (D) μ₀I/(2πa) (E) none of the above.