The most efficient cross-section of sewers in a combined sewerage system is

ANS:B - circular

In a combined sewerage system, which carries both sanitary sewage and stormwater runoff, the most efficient cross-section of sewers is typically considered to be circular. Circular sewers offer several advantages in combined sewerage systems:

1. Hydraulic Efficiency: The circular shape provides optimal hydraulic efficiency by promoting uniform flow distribution and minimizing energy losses. This design helps to prevent sediment deposition and ensures self-cleaning of the sewer.
2. Structural Integrity: Circular sewers exhibit superior structural integrity, with the curved shape distributing external loads more evenly. This makes them more resistant to soil pressure, traffic loads, and other external forces.
3. Flow Capacity: Circular sewers maximize flow capacity relative to their cross-sectional area compared to other shapes like rectangular or parabolic. This allows them to accommodate the combined flow of sanitary sewage and stormwater runoff more effectively, reducing the risk of surcharging and overflow during heavy rainfall events.

While other sewer cross-sections like rectangular or parabolic shapes may be suitable for specific applications or constraints, such as space limitations or aesthetic considerations, circular sewers are generally preferred in combined sewerage systems for their hydraulic efficiency, structural resilience, and flow capacity. Therefore, among the options provided, the most efficient cross-section of sewers in a combined sewerage system is typically considered to be circular.