An optical pin is placed at various positive distances \(d_o\) along the principal axis of a concave spherical mirror with focal length \(f\). The corresponding image distance \(d_i\) is measured, where real images have \(d_i > 0\) and virtual images have \(d_i < 0\). Which of the following graphs best represents image distance \(d_i\) as a function of object distance \(d_o\) for all values of \(d_o > 0\)?