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

1. A
   $1\Omega$
2. B
   $0.5\Omega$
3. C
   $4\Omega$
4. D
   $2\Omega$