What is a trap in plumbing?

• A. A pipe joining method
• B. A device to measure pressure
• C. A type of valve
• D. A P-trap or U-trap that prevents sewer gases from entering the building

Answer: (D)

A trap in plumbing is the water-filled bend in a drain line that creates a barrier to block sewer gases from entering the building. The usual forms are a P-trap or a U-trap, found under sinks and other fixtures. When water sits in the bend, it seals off the sewer by preventing gases from traveling back up the pipe. This keeps the indoor air safe and odor-free. Proper venting and regular water in the trap help maintain that seal and prevent siphoning, which can happen if drainage is too rapid or venting is inadequate. If a trap dries out or is siphoned, sewer gases can escape, which is why traps are designed to be part of a vented drainage system and are kept filled with water. The other options describe different plumbing concepts—joining pipes, measuring pressure, or controlling flow with a valve—and none of these create the water seal that traps provide. So the idea that a P-trap or U-trap prevents sewer gases from entering the building is the correct understanding of what a trap does.