On cold days, it may be necessary to preheat the propane tanks because

• A. The temperature of the liquid propane controls the burner pressure during combustion
• B. The color of the flame changes
• C. Propane freezes at low temperatures
• D. The burner requires more oxygen

Answer: (A)

The main idea here is how propane pressure to the burner depends on temperature. Propane in a balloon or tank is stored as a liquid, and the gas that feeds the burner comes from that liquid vaporizing. The rate at which propane vaporizes—and thus the pressure available to the regulator and burner—depends on temperature. In cold weather, the liquid propane vaporizes more slowly, so the regulator sees a lower gas pressure and the burner may not get enough fuel to maintain a steady flame.

Preheating the tanks warms the propane, increases its vapor pressure, and allows more gas to be drawn off to the burner, helping maintain the required burner pressure and a steady flame. The other choices aren’t the driving reason: flame color isn’t a reliable indicator of pressure, propane’s “freezing” concept isn’t the correct mechanism (its boiling point is far below typical cold days), and the burner’s needs aren’t governed by oxygen supply changing with temperature.