It Has Been Suggested That the Decomposition of NO₂ Occurs $\rarr$

It has been suggested that the decomposition of NO₂ occurs via the following mechanism:
NO₂ $\rarr$ NO + O (Rxn I)
O + NO₂ $\rarr$ 2NO + O₂ (Rxn II)
Predict the rate determining step.

A) Rxn II; this reaction is bimolecular, bimolecular reactions are always slower than unimolecular reactions.
B) Rxn I; unimolecular reactions are always slower than bimolecular reactions.
C) Rxn I; O is highly reactive and thus Rxn II will be very fast.
D) Rxn II; NO₂ is highly reactive and thus Rxn I will be very fast.
E) Rxn I and II are both elementary reactions and will proceed at equal rates.