A proposed mechanism for the decomposition of hydrogen peroxide (H₂O₂) consists of three elementary steps:

H₂O₂(g) → 2OH(g)
H₂O₂(g) + OH(g) → H₂O(g) + HO₂(g)
HO₂(g) + OH(g) → H₂O(g) + O₂(g)

The rate law of the reaction is first order with respect to H₂O₂(g). Which step of the mechanism is the rate-determining step?

A. The first step.
B. The third step.
C. Reactions in the gas phase do not have rate-determining steps.
D. Steps 1 and 2 are both rate-determining steps.
E. The second step.