how does the third law of thermodynamics allow absolute entropies of substances to be determined? a) it defines a reference point by which entropy changes can be measured and assigned as an absolute entropy for a substance. b) it defines the temperature at which all absolute entropies are measured. c) it requires that all energy in a process be conserved to accurately measure an absolute entropy. d) it provides a reference point for the absolute entropy of the universe. e) it requires that a process be reversible to measure the absolute entropy change.