In case of hydraulically smooth pipe, the resistance to flow depends only on the Reynolds number, whereas for a hydraulically rough pipe, the resistance to flow is governed by the relative roughness. Two pipes are said to have the same hydraulic roughness, when they have equal values of___________________?

• A. Relative roughness
• B. Absolute roughness
• C. Friction co-efficient for flows at equal Reynold number
• D. All A., B. & C.