Which of the following is not a method of cross-connection control?

The correct answer identifies gate valves as not a method of cross-connection control. Cross-connection control is essential in preventing contamination of drinking water supplies by ensuring a clear separation between potable water and non-potable water sources.

Air gaps, backflow preventers, and vacuum breakers all serve specific purposes in maintaining that separation. An air gap involves a physical space between the end of a water supply line and the flood level of a plumbing fixture, which prevents backflow due to siphoning. Backflow preventers are devices that specifically allow water to flow in one direction while preventing reverse flow, providing an effective means to eliminate cross-connections. Vacuum breakers help prevent siphoning by allowing air into the piping system if a negative pressure arises, thus breaking the potential for backflow.

In contrast, gate valves are primarily designed to either initiate or shut off the flow of water. They do not specifically prevent backflow or protect against contamination in the same manner as the other methods listed. While gate valves may be used in water supply systems, their function does not contribute to controlling cross-connections, making them unsuitable as a cross-connection control method.