What prevents air from entering a water distribution system when water is in motion?

The concept of preventing air from entering a water distribution system during water movement primarily involves the mechanics of water flow and pressure balance. The correct choice highlights that check valves are primarily designed to allow fluid (in this case, water) to flow in one direction while simultaneously preventing backflow. This mechanism is crucial because if water were to start flowing backward, it could create a drop in pressure that might allow air to infiltrate the system.

Check valves function by closing automatically in the event of reverse flow, which helps to maintain the pressure within the system and thus keeps air from being drawn in. This is particularly important in maintaining the integrity and efficiency of a water distribution system, as the introduction of air can lead to various operational issues, including water hammer and reduced system effectiveness.

The other options, while relevant to different aspects of managing pressure and flow within a water system, do not specifically address the prevention of air from entering the system when water is in motion as effectively as check valves do. Pressure relief valves, for example, are designed to release excess pressure, which does not prevent air entry during normal flow conditions. Surface tension, while a critical property of liquids, does not play a role in preventing air infiltration in a distribution system. Backflow preventers help