In a primary-secondary chilled water system, the primary loop is constant volume, supplying chilled water from the chiller plant. The secondary loop uses variable volume pumping to match distribution system load demand efficiently.

A. Secondary loop (Correct): Where variable flow is applied.

B. Open loop: Not a recognized term in this context.

C. Primary loop: Constant flow design.

D. Cooling tower loop: Separate condenser water system, not chilled water distribution.