Which of the following cannot be used for backflow protection in a high hazard?

In high hazard situations, backflow protection is essential to prevent contaminants from entering the potable water supply. The correct choice in this case is that a dual check valve is not an acceptable method for backflow protection in high hazard applications.

A dual check valve provides a certain level of protection, but its effectiveness is limited when facing significant backpressure or backsiphonage that may occur in high hazard scenarios. Under these conditions, it's crucial to implement more robust protective measures that can handle the potential risks associated with high hazard contaminants.

On the other hand, an air gap creates a complete separation between the water supply and potential contaminants, making it a very reliable method of backflow prevention. Reduced pressure zone (RPZ) devices are specifically designed to handle high hazard situations by maintaining a pressure differential that prevents backflow, therefore they are suitable for use in such applications.