Final answer:
Option B). A gravity furnace lacks a blower motor because it relies on the natural movement of warm air rising and cool air sinking to circulate heat.
Step-by-step explanation:
A blower motor is absent from a gravity furnace. A gravity furnace, in contrast to contemporary forced-air heating systems, uses the natural flow of hot air rising and cool air sinking to distribute warmth throughout the house. A convection loop that distributes energy throughout the space allows for this circulation. The air rises and expands when heated by the furnace; it contracts and falls back to the floor when it cools at the ceiling and outside walls, creating space for more warm air to rise. It doesn't need a blower motor, which is a part of forced-air systems used to force air through the ductwork, because it runs on natural air movement instead of forced air.