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).