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 adsorbs on the catalyst's active site 3) A reacts with B to form C and D. Both C and D are on active sites after they are generated. 4) C desorbs from the active site. 5) D desorbs from the active site. Derive -ra when the surface reaction is the rate-limiting step