A unit mass of an ideal gas at temperature T undergoes a reversible isothermal process from pressure P1 to pressure P2 while losing heat to the surroundings at temperature T in the amount of q. If the gas constant of the gas is R, the entropy change of the gas s during this process is
(a) ∆s = R ln (P2/P1)
(b) ∆s = R ln (P2/P1) - q/T
(c) ∆s = R ln (P1/P2)
(d) ∆s = R ln(P1/P2) - q/T
(e) ∆s = 0