The rate of the heterogenous catalytic reaction A(g) + B(g) → C(g) is given by $$ - {{text{r}}_{text{A}}} = frac{{{text{k}}.{{text{K}}_{text{A}}}.{{text{p}}_{text{A}}}.{{text{P}}_{text{B}}}}}{{1 + {{text{K}}_{text{A}}}.{{text{P}}_{text{A}}} + {{text{K}}_{text{C}}}.{{text{p}}_{text{C}}}}},$$ where K A and K C are the adsorption equilibrium constants. The rate controlling step for this reaction is