Which of the following statements about back-siphoning is true?

Back-siphoning refers to the phenomenon where liquid from a tank can flow back into a water supply due to negative pressure or a drop in pressure within the system. The statement that back-siphoning cannot occur if the end of the fill hose stays above the tank contents is correct because maintaining the fill hose above the fluid level in the tank creates a physical barrier. This positioning prevents the liquid from being drawn back into the hose, which is critical in preventing contamination of potable water supplies during the filling process.

In the context of agricultural practices, ensuring that the filling hose remains above the tank contents is a fundamental safety measure to protect against back-siphoning. This practice is crucial to maintain the integrity of the water supply and to comply with safety regulations regarding pesticide application and handling.

The other statements address scenarios that can lead to confusion about back-siphoning and safety. While it's important to be aware of under what circumstances back-siphoning can occur, the positioning of the hose is the key preventative measure.