The correct answer is: A. Full depth.
Sewer pipes are designed to carry wastewater at full depth. This is because full-depth flow provides the most efficient and effective way to transport wastewater. When wastewater flows at full depth, it creates a smooth, even flow that minimizes turbulence and erosion. This helps to prevent clogging and blockages, and it also helps to reduce the risk of sewer backups.
In addition, full-depth flow helps to ensure that all of the wastewater is properly treated. When wastewater flows at full depth, it is exposed to more of the treatment chemicals and processes, which helps to remove more pollutants and contaminants. This helps to protect the environment and public health.
There are a few reasons why sewer pipes are not designed to carry wastewater at half depth or less. First, half-depth flow can create turbulence and erosion, which can lead to clogging and blockages. Second, half-depth flow does not provide as efficient a way to transport wastewater as full-depth flow. Third, half-depth flow does not expose wastewater to as much of the treatment chemicals and processes as full-depth flow, which can lead to less effective treatment.
Therefore, the correct answer to the question is: A. Full depth.