One mole of hydrogen, assumed to be ideal, is adiabatically expanded from its initial state `(P_(1), V_(1), T_(1))` to the final state `(P_(2), V_(2), T_(2))`. The decrease in the internal energy of the gas during this process will be given by
A. `C_(V)(T_(1)-T_(2))`
B. `C_(P)(T_(1)-T_(2))`
C. `(C_(P)+C_(V))/(2)(T_(1)-T_(2))`
D. `(C_(P)-C_(V))(T_(1)-T_(2))`
