What maximum power can be obtained from a battery of $\mathrm{emf} \mathrm{E}$ and internal resistance $\mathrm{r}$ connected with an external resistance $\mathrm{R}$ ?
(A) $\left(E^{2} / 4 r\right)$
(B) $\left(E^{2} / 3 r\right)$
(C) $\left(E^{2} / 2 r\right)$
(D) $\left(\mathrm{E}^{2} / \mathrm{r}\right)$