During pyrolysis of alkanes, $\mathrm{C}-\mathrm{C}$ bond breaks rather than $\mathrm{C}-\mathrm{H}$ bond breaks because:

$\mathrm{C}-\mathrm{H}$ bond is reactive site in alkane

bond dissociation energy of $\mathrm{C}-\mathrm{C}$ bond is lower than that of $\mathrm{C}-\mathrm{H}$ bond.

energy of activation of $\mathrm{C}-\mathrm{C}$ bond is very high

None of the above