In the circuit shown, the current through the 4$ \Omega$ resistor is 1 amp when the points P and M are connected to a d.c voltage source. The potential difference between the points M and and N is :

Question

$\begin {array} {1 1} (1)\;3.2 \: volt & \quad (2)\;1.5\: volt \\ (3)\;1.0\: volt & \quad (4)\;0.5\: volt \end {array}$