$50V$ battery is supplying a current of $10A$ when connected to a resistor. If the efficiency of the battery at this current is 25%, then the internal resistance is:

Question

$\begin {array} {1 1} (A)\;2.5\Omega & \quad (B)\;3.75\Omega \\ (C)\;1.25\Omega & \quad (D)\;5\Omega \end {array}$