7. The elementary chemical reaction
O + ClO → Cl + O2
is made pseudo-first order in oxygen atoms by using a large excess of ClO radicals. The rate constant for the reaction is 3.5 × 10⁻¹¹ cm³/molecule·s. If the initial concentration of ClO is 1.0 × 10¹¹ molecules/cm³, how long will it take for the oxygen atoms to decrease to 10.% of their initial concentration?
(A) 3.2 × 10⁻³ s
(B) 0.66 s
(C) 0.017 s
(D) 23 s
(E) 2.4 s