A flat, tightly wound spiral coil lies in a single plane and consists of \(N\) turns of fine wire. The spiral has an inner radius \(a\) and an outer radius \(b\), with the turns distributed uniformly across the radial distance from \(a\) to \(b\). A steady current \(I\) flows through the coil. Which of the following expressions gives the magnitude of the magnetic field at the center of the spiral coil?