Which of the following is a propagation step in the free-radical bromination of methane?
1. CH$_{3}$• + Br$_{2}$ → CH$_{3}$Br + Br•
2. CH$_{3}$• + CH$_{3}$• → CH$_{3}$CH$_{3}$
3. Br• + Br• → Br$_{2}$
4. CH$_{3}$• + Br• → CH$_{3}$Br
5. Br$_{2}$ $\xrightarrow{hv}$ 2 Br•