For an ideal gas that is compressed isothermally from 20.0 L to 5.0 L, which of the following best describes the sign of the entropy change, ΔS, for the gas?
a) ΔS is zero because the temperature is unchanged.
b) ΔS is positive because the molecules collide with the walls more frequently in the smaller volume.
c) ΔS is negative because the gas occupies a smaller volume.
d) ΔS cannot be determined without knowing the amount of heat exchanged.