18) What is the only completely reliable method for preventing backflow A. Air gap B. Ball valve C. Cross -connection D. Vacuum breaker

The correct answer is A. Air gap.<br /><br />An air gap is a physical separation between the free-flowing discharge of a faucet and any potential source of contamination. It is considered the most reliable method for preventing backflow because it provides a physical barrier that prevents contaminated water from flowing back into the potable water system. The air gap must be at least 6 inches (15 cm) between the faucet outlet and the flood level rim of the receiving vessel to be effective.