# Two sources of equal emf are connected to an external resistance R. The internal resistance of the two sources are $R_1\: and \: R_2 (R_2 > R_1)$. If the potential difference across the source having internal resistance $R_2$ is zero then,
$\begin {array} {1 1} (A)\;R = R_2 \times (R_1+R_2)/R_2-R_1) & \quad (B)\;R=R_2-R_1 \\ (C)\;R=R_1R_2/(R_1+R_2) & \quad (D)\;R=R_1R_2/(R_2-R_1) \end {array}$