Why circular sections are preferred for sewers: Identify the most comprehensive reason set for selecting a circular cross-section in gravity sewers.

Introduction / Context:
The choice of sewer cross-section affects hydraulic capacity, self-cleansing, constructability, and cost. Circular sections are most common for small to medium diameters in municipal systems.

Given Data / Assumptions:

• Gravity sewer applications with standard construction methods.
• Comparison of geometric and economic attributes.

Concept / Approach:
Among shapes, a circle offers a high area-to-perimeter ratio, which implies a larger hydraulic radius R = A/P and, hence, lower resistance for a given perimeter. Circles are also structurally efficient and straightforward to manufacture and install using pipes, leading to cost effectiveness.

Step-by-Step Solution:
Geometric efficiency: For given perimeter, circle maximizes enclosed area.Hydraulic implication: Larger hydraulic mean depth (A/P) yields better flow capacity.Practicality: Circular pipes are widely available, easy to lay, and economical.Therefore, the inclusive choice captures the full rationale.

Verification / Alternative check:
Textbook derivations show the circle maximizes area for a given perimeter, which correlates with a favorable hydraulic radius, and industry costing demonstrates economies of scale for circular pipe products.

Why Other Options Are Wrong:
Single-attribute options do not reflect the totality of hydraulic and economic advantages.“Only because it is easy to clean” is not the primary or sole reason.

Common Pitfalls:

• Assuming non-circular shapes are never used; egg-shaped sections can be advantageous for combined sewers.
• Ignoring partial flow hydraulics where circular conduits maintain reasonable performance.

Final Answer:
All of the above