Why is the vent terminal of a cistern designed to point down toward the ground?

The correct choice reflects a critical design element in plumbing systems. The vent terminal of a cistern is engineered to point downward to prevent precipitation, such as rain or snow, from entering the vent. This is crucial because any moisture entering the vent can lead to the accumulation of water in the vent line, potentially causing blockages or issues with airflow. By directing the vent terminal downward, the design helps ensure that any water that might collect is directed away from the cistern and prevents it from interfering with the functioning of the plumbing system.

The downward orientation also plays a role in maintaining the internal pressure balance of the cistern, ensuring that gases can vent without obstruction, which is essential for proper drainage and preventing any adverse effects on the system's performance. This design consideration ultimately fosters a more reliable and efficient operation of the plumbing system.