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

• A

  $4\Omega$

• B

  $2\Omega$

• C

  $1\Omega$

• D

  $0.5\Omega$