A thin, flat, nonconducting disk of radius \(R\) lies in the \(xy\)-plane with its center at the origin. A total positive charge \(Q\) is distributed uniformly over the surface of the disk. Which of the following expressions correctly gives the electric potential \(V(z)\) along the positive \(z\)-axis at a distance \(z\) from the center of the disk, relative to zero potential at infinity?