Final answer:
The correct answer is D. Vent, flue gases, lighter, as natural gas is less dense than air and will rise through the vent, releasing flue gases if the pilot fails to light on a furnace.
Step-by-step explanation:
If the pilot light fails on a natural gas furnace, the gas will rise through the vent and vent flue gases since natural gas is lighter than air. The correct response to fill in the blanks would be D. Vent, flue gases, lighter. This is because natural gas, chiefly composed of methane, has a lower density compared to air, causing it to rise when unburned. In contrast, gases like propane or butanes are heavier than air and would collect near the floor, posing a different type of hazard.
Methane, when it burns, creates hot gas which tends to rise due to its reduced density as per the ideal gas law, explaining the venting process in furnaces. Unburned methane is also a concern for fire hazards as its accumulation could lead to explosions; hence safety measures are put in place to ensure any unburned gas is safely vented away from ignition sources.