12. The Fig. 6 shows the cross section of a long cylindrical conductor with radius a, which contains a long cylindrical hole of radius b. The central axes of the cylinder and the hole are parallel and are distance d apart. Current density j is uniformly distributed in the conductor with its direction along the z-axis. What is the magnetic field B in the hole? where j = |j| is the magnitude of the current density, and eₓ, eᵧ, and eₓ are the unit vectors along x, y, and z-axes. (A) (B) (C) (D) (E)