What must be bonded to an electrical grounding system in terms of metal gas lines?

The correct answer focuses on the need to bond furnace gas lines to the electrical grounding system due to safety concerns. Bonding ensures that any stray electrical currents that could potentially energize metal structures, such as gas lines, are safely directed into the ground, minimizing the risk of electric shock or fire.

Furnace gas lines are particularly important to bond because they may carry gas that can support combustion and could pose a significant hazard if an electrical fault occurs. Proper bonding prevents the buildup of voltage on the gas piping, which can be a consequence of induction or fault conditions. As a result, bonding these components creates a safer environment within a building's electrical system.

While gas meters and gas piping systems also play roles in safety, the specific bonding requirement is often emphasized for gas lines connected to appliances that are subject to more direct interaction with electrical systems, like furnaces, which have components that may potentially introduce electrical faults. Therefore, while the bonding of various gas components is crucial, the integration of furnace gas lines into the grounding system is particularly stressed for electrical safety protocols.