In the event of a fire, the doors to a stair tower must be closed to prevent the spread of fire by____________.

Correct Answer: Option D — convection

Convection is the transfer of heat through the movement of hot gases and smoke. Stair towers act as chimneys; if doors are left open, the rising heat and smoke will rapidly spread the fire to upper decks. Closing these doors maintains fire integrity and protects escape routes for the crew.