Trench ventilation operations must be completed before:

Trench ventilation operations are critical in controlling an ongoing fire, especially in structures with heavy fire involvement and significant smoke movement. The correct answer highlights that these operations must be completed before the fire reaches the trench.

When a fire is advancing and gets to the trench, there's a significant risk of fire spread and increased heat release rates due to freshly introduced oxygen. If a firefighter were to conduct trench ventilation after the fire has reached the trench, it could result in a rapid intensification of the fire, creating extreme hazards for both firefighters and any occupants who may still be inside the structure.

Successful trench ventilation creates an effective pathway for smoke and heat to escape, allowing for safer and more effective firefighting operations. This proactive measure should be taken before the fire can reach the trench area to mitigate further risks.

Understanding this concept is vital for firefighters to ensure they maintain control over the fire's behavior and enhance the safety of the operation.