In a potentiometer experiment the balancing with a cell is at length 240 cm. On shunting the cell with a resistance of $2 \Omega$ the balancing length becomes 120 cm. The internal resistance of the cell is

Question

$\begin {array} {1 1} (A)\;1\Omega & \quad (B)\;0.5\Omega \\ (C)\;4\Omega & \quad (D)\;2\Omega \end {array}$