Two cylinders 'A'and 'B' fitted with pistons contain equal number of moles of an ideal mono-atomic gas at $400\;K$. The piston of 'A' is free to move while that of 'B' is held fixed. Same amount of heat energy is given to the gas in each cylinder. If the rise in temperature of the gas in 'B' is :

Question

\[\begin {array} {1 1} (1)\;21\;K & \quad (2)\;35\;K \\ (3)\;42\;K & \quad (4)\;70\;K \end {array}\]