2a(g) + b(g) ⇄ 2 c(g)
when the concentration of substance b in the reaction above is doubled, all other factors being held constant, it is found that the rate of the reaction remains unchanged. the most probable explanation for this observation is that:

(a) the order of the reaction with respect to substance b is 1.
(b) substance b is not involved in any of the steps in the mechanism of the reaction.
(c) substance b is not involved in the rate-determining step of the mechanism but is involved in subsequent steps.
(d) substance b is probably a catalyst, and as such, its effect on the rate of the reaction does not depend on its concentration.
(e) the reactant with the smallest coefficient in the balanced equation generally has little or no effect on the rate of the reaction.