Consider the reaction mechanism shown below:
A (g) +* (s) ↔ A * (ads)
B (g) + A * (ads) ↔ C * (ads) (SLOW)
A * (ads) + C* (ads) ↔ D * (ads) +* (s)
D * (ads) ↔ D (g) + * (s)
where *(s) represents an empty active site of the catalyst. Obtain the reaction rate equation.