Before a gas container can be used, what must be attached to each of the control valves?

The correct choice is a regulator because it plays a critical role in managing the pressure of gas as it flows from the container. Gas containers store substances under high pressure, and the regulator helps reduce this pressure to a safe and usable level. By controlling the gas flow, the regulator ensures that the system operates efficiently and safely, preventing potential hazards associated with high-pressure gas release.

Using a regulator is essential for maintaining system integrity and functionality. Without it, the gas would be released at an unsafe pressure, leading to possible leaks or ruptures in the piping system. Proper installation of the regulator on each control valve is vital to ensure safe and effective operation in any gas piping and storage system.

While each of the other options represents important components in the context of gas systems, they do not serve the specific function of adjusting and managing the pressure at the control valves, which is the primary requirement before a gas container is utilized.