When should first due companies lay their own supply line at a structure fire?

First due companies should lay their own supply line at a structure fire when there is clear evidence of a working fire. This choice is correct because establishing a supply line ensures that adequate water is readily available to fight the fire effectively and safely. When there is clear evidence of a working fire, such as visible flames or significant smoke, it indicates an urgent need for firefighting efforts. Delaying the establishment of a supply line in such situations could lead to inadequate water supply, hampering the firefighting operations and increasing the risk to life, property, and the firefighters themselves.

The timing of laying a supply line is critical, as it directly impacts the effectiveness of the response. Options suggesting conditions like assessing the fire location or proximity to hydrants may lead to delays in initiating crucial firefighting activities. The presence of a second pumper is not a prerequisite for first due companies to establish their supply; they must be prepared to act independently based on the fire conditions they encounter.