An ideal parallel-plate capacitor has plate area \(A\), plate separation \(d\), and capacitance \(C_0\) when the space between the plates is filled with air. A slab of dielectric material with dielectric constant \(\kappa\) and thickness \(d/2\) is inserted into the capacitor such that its faces are parallel to the conducting plates, leaving the remaining half of the gap filled with air. Which of the following expressions correctly gives the new equivalent capacitance \(C\) of the capacitor in terms of \(C_0\) and \(\kappa\)?