Final answer:
A backflow prevention device always prevents cross-contamination in the water supply, to maintain safe drinking water regardless of local regulations or the setting.
Step-by-step explanation:
The question of whether a backflow prevention device prevents cross-contamination in the water supply is a health and safety concern. The correct answer to the question is A) Always. Backflow prevention devices are designed to prevent unsafe water from reversing flow and entering the clean water supply. This is crucial because, as indicated in Figure 3.19(a) and Figure 6.13(a), contaminants at trace-level concentrations can render unfiltered tap water unsafe for drinking and cooking. In contrast, as mentioned in Figure 3.19(b) and Figure 6.13(b), inline water filters are used to reduce the concentration of solutes in tap water, but they serve a different function than backflow preventers. While local regulations may dictate the type and rigor of backflow prevention required, the purpose of these devices is fundamentally to always prevent cross-contamination to maintain safe drinking water.