A long cylindrical capacitor of length \(L\) consists of a solid inner conducting cylinder of radius \(a\) and a thin coaxial outer conducting cylindrical shell of radius \(b\), where \(L \gg b\). The space between the conductors is evacuated. A total charge \(+Q\) resides on the inner cylinder and \(-Q\) resides on the outer shell. Which of the following expressions gives the capacitance per unit length \(\dfrac{C}{L}\) of this capacitor in terms of \(a\), \(b\), and fundamental constants?