Which of the following factors can cause contaminants to degrade in water distribution systems?

The chosen answer identifies several significant factors that can lead to the degradation of contaminants in water distribution systems. Temperature plays a critical role in biological activity; higher temperatures can accelerate the growth of microorganisms that can break down certain contaminants. Flow in the distribution system influences how long water, and any associated contaminants, remains in the pipe system, with stagnant water often allowing for greater contamination and degradation. Water age refers to the time water spends in the distribution system; longer water age can lead to increased degradation of organic contaminants due to microbiological activity and chemical changes. Lastly, dead ends in piping can create areas with slow or no flow, leading to stagnation and potential buildup or degradation of contaminants.

Other factors mentioned in the remaining options contribute in different ways but do not comprehensively cover the dynamics involved in contaminant degradation in the manner that option B does. For instance, while UV exposure and chlorine levels (mentioned in the first option) do influence water quality, they do not directly pertain to degradation in the same interconnected manner as flow and water age. The presence of biofilms, pipe materials, and age highlighted in the last option can also affect the system's overall quality but are not as central to the specific degradation process as the factors in option B.