Final answer:
Fuel-oil tanks are filled by a pump from a primary storage tank located on the lowest floor, allowing gravity to aid in fuel transfer to higher tanks within buildings.
Step-by-step explanation:
Fuel-oil tanks must be filled by a pump supplied from a primary storage tank located on the lowest floor. This is because the position of the primary storage tank on the lowest floor takes advantage of gravity, thereby requiring less energy for the pump to transfer the fuel to the tanks situated at a higher elevation. In many systems, these pumps are part of a larger fuel oil transfer system, which includes pipes, valves, and control mechanisms to manage the flow of oil and maintain a supply for the building's use.