Before water enters public pipelines, small amounts of _____ are added to kill bacteria.

Water treatment plants add small amounts of chlorine to public pipelines because chlorine is highly effective in killing bacteria and other harmful microorganisms found in raw water. Oxygen, fluoride, and iodine are not commonly used to disinfect water because they do not have the same strong disinfectant properties as chlorine. Oxygen is already present in water and does not effectively kill microorganisms, fluoride is added primarily for dental health reasons, and iodine is not effective at killing bacteria in low doses. Therefore, option B is the only viable and effective solution for eliminating harmful bacteria from water in public pipelines.