An ideal parallel-plate capacitor with plate area \(A\) and initial plate separation \(d_0\) is charged to a magnitude of charge \(Q_0\) and then disconnected from the charging source. The separation between the plates is then increased to a distance \(d > d_0\). Which of the following best describes the graph of the potential difference \(V\) across the capacitor as a function of the plate separation \(d\)?