One mole of helium is adiabatically expanded from its initial state `(P_(i),V_(i),T_(i))` to its final state `(P_(f),V_(f),T_(f))`. The decrease in the internal energy associated with this expansion is equal to
A. `C_(V)(T_(i)-T_(f))`
B. `C_(P)(T_(i)T_(f))`
C. `(1)/(2)(C_(P)+C_(V))(Ti-Tf)`
D. `(C_(p)+C_(V))(T_(i)-T_(f))`
