Which step is chain propagation step in the following mechanism ?
i) ${\mathrm{Cl}}_2\underset{ }{\overset{\mathrm{h\upsilon }}{\to }}{\mathrm{Cl}}^{·}+{\mathrm{Cl}}^{·}$

ii) ${\mathrm{Cl}}^{·}+{\mathrm{CH}}_4\underset{ }{\overset{ }{\to }}{\mathrm{\hspace{0.17em}}}^{·}{\mathrm{CH}}_3+\mathrm{HCl}$
iii) ${\mathrm{Cl}}^{·}+{\mathrm{Cl}}^{·}\underset{ }{\overset{ }{\to }}{\mathrm{Cl}}_2$
iv) ${\mathrm{CH}}_3+{\mathrm{Cl}}^{·}\underset{ }{\overset{ }{\to }}{\mathrm{CH}}_{\mathrm{3}}\mathrm{Cl}$