The reaction A +B C+D is carried out over a solid catalyst. After reactant A and B transport into the catalyst particles, the following steps happen: 1) A adsorbs on the catalyst's active site 2) B does not adsorbs on the catalyst's active site and remain in the fluid phase 3) A, which is adsorbed on the active site, reacts with B in the fluid phase to form C and D Only C is on active sites after it is generated. D is not on the active site but travels immediately to the fluid phase. 4) C desorbs from the active site. Derive - rA when the surface reaction is the rate-limiting step