The hydrogen abstraction step by $\mathrm{Cl}{}^{\bullet }$ from an alkane is only weakly selective between primary, secondary and tertiary C-H bonds. According to the Hammond postulate the reason is that

1. Athe C-H bonds of an alkane have identical bond energies
2. Bthe step is exothermic, so the transition state is early and resembles the reactants, with little radical character developed on carbon
3. Cthe step is endothermic, so the transition state is late and resembles the alkyl radical
4. D$\mathrm{Cl}{}^{\bullet }$ is too bulky to distinguish the C-H bonds