The adiabatic expansion of an ideal gas is described by the equation PVY = C, where y and C are constants. The work done by the gas is expanding adiabatically from the state (Vi, P) to (Vf, Pf) is equal to A. PfVf B. 1/2(Pi + Pf)(Vf - Vi) C. (PfVf - PV)/(1-y) 1+ D. P(V Y-V+ Y)/(1+y) E. Pf(VfY-V)/(1+y)