Which of the following statement is/are correct? (a) A catalyst is used in the rate determining step of a mechanism, but then is formed again in some subsequent step so that there is no change in the concentration of the catalyst during the course of the reaction. (b) Catalyst does not appear in the overall stoichiometric equation for the reaction. (c) The role of the catalyst is to provide an alternate activated complex, one with a lower activation energy. (d) All are correct.