for a gas at fixed temperature, pressure and particle number (t, p, n) the natural thermodynamic potential is g = u - ts pv where u is the internal energy, v is volume and s is entropy. which of the following is the corresponding potential for a gas at fixed (T, V, μ)

a. G + TS
b. G - PV
c. G - PV + μN
d. G - PV - μN