A long, hollow cylindrical conductor has an inner radius \(a\) and an outer radius \(b\). The conductor carries a total current \(I_0\) directed along its length parallel to the central axis. Within the conducting material (\(a \le r \le b\)), the volume current density is non-uniform and given by \(J(r) = \dfrac{C}{r}\), where \(C\) is a constant and \(r\) is the radial distance from the central axis. Which of the following expressions represents the magnitude of the magnetic field \(B(r)\) as a function of radial distance \(r\) in the region \(a < r < b\)?