Which backflow prevention method is considered the most reliable because it creates a physical gap that no back-siphonage can cross?

解析

An air gap, an unobstructed vertical space between a water outlet and the flood rim of the receiving fixture, is the most reliable backflow prevention because there is no physical connection for contaminated water to travel back through. The gap should generally be at least twice the diameter of the supply opening and never less than one inch. A closed valve and a tight gasket only hold pressure and can fail or be left open, and a hose resting in a mop bucket is the cross-connection an air gap exists to prevent.