One mole of a monoatomic ideal gas expands adiabatically at initial temperature T against a constant external pressure of 1atm from 1 litre to 3 litres. The final temperature of the gas is
A T
B T⁽⁵ᴿ/²⁾ / 3
C T−2/(1.5×0.082)
D T+2/(1.5×0.082)