A long, hollow cylindrical conductor has an inner radius \(a\) and an outer radius \(b\). A total current \(I\) is distributed uniformly across the conductor's cross-sectional area between \(r = a\) and \(r = b\), flowing parallel to the central axis. Which of the following expressions gives the magnitude of the magnetic field \(B(r)\) at a radial distance \(r\) from the central axis in the region \(a < r < b\)?