During the free radical chlorination of methane, which of the following reactions has the lowest collision frequency?
a) Cl• + •CH₃ → CH₃Cl
b) Cl • + Cl₃ → Cl₃ + Cl•
c) CH₃ +CH₄ → CH₄ + •CH₃
d) CH₃ + Cl₃ → CH₃Cl + Cl•