Which statement best describes the mechanism by which enzymes catalyze a reaction? a. enzymes absorb the change in enthalpy (δh), permitting a reaction to proceed. b. enzymes lower the concentration of the substrates, thereby driving the equilibrium to favor product formation. c. enzymes are completely used up in the course of the reaction and therefore favor product formation. d. enzymes lower the energy barrier necessary to achieve the transition state of a reaction. e. enzymes lower the concentration of the products, thereby driving the reaction to favor product formation.