NBS is used to substitute a bromine atom for a hydrogen atom at an allylic or benzylic carbon atom. The source of the bromine atom, molecular bromine, is produced in low concentrations by NBS through a mechanism involving movement of both single electrons and pairs of electrons. Provide the missing arrows in the boxes to show how NBS (N-bromosuccinimide) reacts with an alkene to substitute a bromine atom for a hydrogen atom at an allylic (or benzylic) carbon.