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.