In the study of reaction mechanisms, which statement best defines a reaction intermediate?
a) A species that lowers the activation energy of a reaction and is regenerated unchanged, so it appears in both the first and last elementary steps
b) A short-lived high-energy arrangement of atoms present at the maximum of the potential-energy profile for an elementary step
c) A species that is formed in one elementary step and consumed in a later elementary step, so it cancels and is absent from the overall balanced equation
d) A reactant in the overall balanced equation that momentarily increases in concentration during a fast pre-equilibrium before being used up in the slow step