One mole of monoatomic ideal gas at T (K) is expanded from 1 L to 2 L adiabitcally under constant external pressure of 1 atm. The final temperature of gas in kelvin is

Question

(a) T
(b) $\frac{T}{2^{5/3-1}}$
(c) T - $\frac{2}{3\times 0.0821}$
(d) T + $\frac{2}{3\times 0.0821}$