One mole of an ideal monoatomic gas at temperature `T` and volume `1L` expands to `2L` against a constant external pressure of one atm under adiabatic conditions, then final temperature of gas will be:

A. `T+(2)/(3xx0.0821)`

B. `T-(2)/(3xx0.0821)`

C. `(T)/(2^(5//3-1))`

D. `(T)/(2^(5//3+1))`