In a potentio meter experiment, the balancing length with a cell is at a length of 240 cm. On shunting the cell with a resistance of 2 $\mathrm{\Omega }$, the balancing length  becomes 120cm. The internal resistance  of the cell is:

• A

  4 $\mathrm{\Omega }$

• B

  2 $\mathrm{\Omega }$

• C

  1 $\mathrm{\Omega }$

• D

  0.5 $\mathrm{\Omega }$