A cell supplies a current $\mathrm{I}$, through a resistance $\mathrm{R}_{1}$ and a current $\mathrm{I}_{2}$ through a resistance $\mathrm{R}_{2}$ the internal resistance of a cell is...
(A) $\mathrm{R}_{2}-\mathrm{R}_{1}$
(B) $\left\{\left(\mathrm{I}_{1}+\mathrm{I}_{2}\right) /\left(\mathrm{I}_{1}-\mathrm{I}_{2}\right)\right\} \mathrm{R}_{1} \mathrm{R}_{2}$
(C) $\left\{\left(\mathrm{I}_{1} \mathrm{R}_{2}-\mathrm{I}_{2} \mathrm{R}_{1}\right) /\left(\mathrm{I}_{1}-\mathrm{I}_{2}\right)\right\}$
(D) $\left\{\left(\mathrm{I}_{2} \mathrm{R}_{2}-\mathrm{I}_{1} \mathrm{R}_{1}\right) /\left(\mathrm{I}_{1}-\mathrm{I}_{2}\right)\right\}$