What can happen if a plumbing system is not properly vented?

The correct answer highlights a critical aspect of plumbing systems: proper venting is essential for maintaining appropriate drainage and trap function. When a plumbing system lacks adequate venting, it can cause slow drainage because air pressure may not equalize properly within the system. This condition can create a vacuum that interferes with the water flow, slowing down the drainage process.

Furthermore, improperly vented systems can lead to siphoning of traps, which is when the water seal in a trap is drawn out. This occurs because the vacuum created can pull water out of the trap, allowing sewer gases to enter the living space, which poses health hazards and unpleasant odors. Overall, proper venting ensures that drainage occurs smoothly and efficiently, while protecting the functionality of traps and maintaining air pressure in the system.