Final answer:
The principal heating system heats the entire house, whereas the supplementary heating system provides additional warmth, typically in specific areas that need extra heat.
Step-by-step explanation:
The difference between a principal and a supplementary heating system is that the principal heating system is designed to heat the entire house, while the supplementary heating system provides extra warmth to a home. Option A is the correct choice. The principal heating system could be a furnace, a heat pump, or any other central heating infrastructure that manages the overall heating needs of the space. On the other hand, a supplementary heating could be a space heater, fireplace, or any additional heat source used to augment the principal system, often targeted to heat specific areas that require extra warmth. Supplementary systems are not typically meant to maintain comfortable temperatures throughout the entire house by themselves.