A thin, flat, nonconducting annular disk lies in the \(xy\)-plane centered at the origin. The disk has an outer radius \(R\), a central circular hole of radius \(\dfrac{R}{2}\), and carries a uniform surface charge density \(\sigma\). A point \(P\) is located on the positive \(z\)-axis at a distance \(z\) from the origin. Which of the following expressions gives the magnitude of the electric field at point \(P\)?