A spherical capacitor consists of two concentric spherical conductors, where outer and inner radii are $r_1$ and $r_2$ respectively as shown in the figure.The capacitance of the spherical capacitor is

Question

$\begin{array}{1 1} \text{a. $c = \frac{4 \pi \varepsilon_o r_1 r_2 }{r_1+r_2}$} \\ \text{b. $c = \frac{4 \pi \varepsilon_o r_1 r_2 }{r_1-r_2}$} \\ \text{c. $c = \frac{4 \pi \varepsilon_o r_1 r_2 }{r_2-r_1}$} \\ \text{d. $c = \frac{4 \pi \varepsilon_o (r_1 r_2) }{r_1r_2}$} \end{array}$