A long, flat copper ribbon of width \(w\) lies in the \(xy\)-plane, centered on the \(y\)-axis between \(x = -w/2\) and \(x = w/2\). The ribbon carries a total current \(I\) distributed uniformly across its width in the \(+y\)-direction. Which of the following expressions gives the magnitude of the magnetic field \(B\) at a point on the \(z\)-axis at a distance \(z\) above the center of the ribbon?