Internal resistance of a given cell is to be found by a potentiometer and the circuit is as shown in the figure. When switch S is open balance length is found to be 250 cm and when S is closed, balance length is found to be 210 cm. The internal resistance r is

 

 

• A

  $1.5\Omega$

• B

  $0.72\Omega$

• C

  $1.9\Omega$

• D

  $1.2\Omega$