Which of the following statements best describes how the concentration of a homogeneous catalyst can appear in the experimentally determined rate law for the catalyzed conversion of reactant R to product P?
a) Because the catalyst is regenerated, its concentration never appears; the reaction is always zero order in catalyst.
b) The catalyst appears with a negative order because it is consumed and then regenerated during the reaction.
c) The catalyst may appear with a positive order when it participates in the slow elementary step of the mechanism.
d) The catalyst appears in the rate law only when it is in a different phase from the reactants.