Dampers in flues must be constructed to ensure they cannot completely ____ the passage of flue gases at any time.

The correct choice indicates that dampers in flues must be designed to prevent the complete cutting off of the passage of flue gases at any time. This is critical because proper ventilation is essential for the safe operation of oil burners and heating systems. If flue gases are completely severed from their pathway, it could lead to dangerous conditions such as carbon monoxide buildup or even system failure.

The design requirement ensures that even when the dampers are in the closed position, there is still a controlled pathway for some gas to escape, reducing the risk of pressure buildup and facilitating adequate ventilation. Not allowing flue gases to be completely cut off contributes to better safety and efficiency of heating systems, ultimately protecting both equipment and occupants.

Other options do not convey this crucial aspect of flue damper function and can imply different levels of obstruction that may not necessarily address the safety concerns associated with complete blockage of flue gas passage.