You are releasing carbon dioxide gas (CO2) into an engine compartment to extinguish a fire. The CO2 will be most effective if the __________.

Correct Answer: Option D — compartment is closed and airtight

Fixed CO2 systems rely on reducing oxygen concentration below 15 percent. Airtight boundaries prevent the dense gas from escaping and fresh air from entering, ensuring the smothering agent maintains effective concentration throughout the compartment.