An air-filled parallel-plate capacitor has plate area \(A\), plate separation \(d\), and initial capacitance \(C_0\). An uncharged conducting metal slab of thickness \(t\) (where \(t < d\)) and area \(A\) is inserted into the space between the plates, positioned parallel to the plates without touching either plate. Which of the following expressions represents the new capacitance \(C'\) of the capacitor system in terms of \(C_0\), \(d\), and \(t\)?