How long will it take for the oxygen atoms to decrease to 10.% of their initial concentration

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•10^11 molecules/cm3, how long will it take for the oxygen atoms to decrease to 10.% of their initial concentration?
a. 2.4 s
b. 0.017 s
c. 3.2•10–3 s
d. 0.66 s
e. 23 s

the answer is d any idea how you would solve this question?

Hmmm looks like you are a chemistry or ChemE major?
use 1st order kinetic:
d[O]/dt = -k[O][ClO], separate variables now
d[O]/[O] = -k[ClO]dt, integrate
ln([O]/[Oi]) = -k[ClO]t, plug in numbers
ln(0.1/1) = -(3.5x10^-11)(1x10^11)t
=> t = 0.66s
Good luck with your studies!