Why are duplicate (parallel) pipes laid in a water-supply network? Choose the most complete reason set reflecting planning, operation, and reliability considerations.

Introduction / Context:
Parallel mains or duplicate feeders are common in transmission and distribution systems. They bolster capacity, improve reliability during maintenance, and provide flexibility to handle growth in demand or fire flows. Understanding these motives improves network design and asset management planning.

Given Data / Assumptions:

• Urban system with variable daily and seasonal demands.
• The utility aims to maintain supply during maintenance or failures.
• Capital planning targets both present and future needs.

Concept / Approach:
Hydraulically, flow capacity in a pipe scales roughly with diameter^5 in the Hazen–Williams framework for a given headloss and length. Adding a parallel main lowers headloss and increases total conveyance. Operationally, isolation of one main for repair while the other remains in service increases reliability. Strategically, dual mains help meet peak and fire demands without excessive velocities or pressure drops.

Step-by-Step Solution:
Capacity: a second main shares the flow, reducing headloss and velocity.Maintainability: one line can be isolated while the parallel line maintains supply.Future-proofing: provides redundancy and headroom for growth and emergencies.

Verification / Alternative check:
Many large utilities specify dual feeders from treatment plants to major service areas, with sectionalizing valves to manage outages and maintain minimum service pressures.

Why Other Options Are Wrong:

• Single motives (A, B, or C alone) are true but incomplete.
• “To reduce water age only” is too narrow; while lower residence time can result, it is not the primary reason.

Common Pitfalls:

• Assuming a single oversized main is always better; redundancy often outperforms a monolithic asset.
• Ignoring operational constraints such as surge, valve control, and energy costs.

Final Answer:
all the above