When work is performed on piping that contains or has contained gas, what precaution must be taken?

The requirement to eliminate all sources of ignition before beginning work on piping that contains or has contained gas is crucial for ensuring safety. Gas lines can contain flammable gases that pose a significant explosion and fire risk when exposed to an ignition source. By removing any potential sources of ignition, such as open flames, electrical sparks, or static electricity, the risk of an accident occurring is greatly reduced. This precaution helps ensure a safe working environment for personnel and protects the integrity of the facility.

The other choices involve practices that could increase the risk of explosion or fire, which emphasizes the importance of adhering to strict safety protocols when dealing with potentially hazardous materials. It's critical in industrial settings that safety measures are prioritized to prevent catastrophic incidents.