A spherical capacitor consists of a solid conducting sphere of radius \(a\) surrounded by a thin concentric conducting spherical shell of radius \(b\) (where \(b > a\)). The space between the inner sphere and outer shell is a vacuum. Which of the following correctly gives the capacitance \(C\) of this spherical capacitor, as well as the limiting capacitance \(C_\infty\) as the outer radius \(b\) approaches infinity?