A thin, flat, nonconducting annular disk lies in the xy-plane centered at the origin, with inner radius a and outer radius b, where 0 < a < b. The disk carries a uniform positive surface charge density \(\sigma\). The electric field magnitude \(E(z)\) is evaluated at points along the positive z-axis. Which of the following pairs correctly identifies the leading functional dependence of \(E(z)\) on distance z in the near-center regime (\(0 < z \ll a\)) and in the far-field regime (\(z \gg b\))?