Which section of piping is located at the lowest point of a building's drainage system and receives discharges from interior drainage?

• A. Building Sewer
• B. Soil Stack
• C. Trap Arm
• D. Building Drain

Answer: (D)

The main idea is identifying which part sits at the bottom of the interior drainage network and collects the waste from inside the building. The building drain is the section that runs at the lowest point inside the building and receives discharges from interior drainage, then carries it onward to the building sewer. The soil stack is the vertical main that carries waste downward through the building, but it is not the lowest point where interior drains converge. The trap arm is a short horizontal run from a fixture trap toward the waste system and does not serve as the collection point for all interior drainage. The building sewer carries the wastewater from the building drain to the public sewer, which is outside the building, so it is downstream of the building drain. Therefore, the building drain is the correct element.