Chlorine atoms catalyze the stratospheric conversion of ozone to oxygen. One elementary step in a proposed mechanism is
(1) Cl + O₃ → ClO + O₂
Which additional single elementary step would complete a two-step mechanism that both regenerates Cl and gives the overall reaction O₃ + O → 2 O₂ ?
a) ClO + O₃ → Cl + 2 O₂
b) ClO + O₂ → Cl + O₃
c) ClO + O → Cl + O₂
d) ClO → Cl + O