The flow of water (mass density = 1000 kg/m3 and kinematic viscosity = 10-6 m2/s) in a commercial pipe, having equivalent roughness ks as 0.12 mm, yields an average shear stress at the pipe boundary = 600 N/m2. The value of ks/δ' (δ' being the thickness of laminar sub-layer) for this pipe is

• 0.25
• 0.50
• 6.0
• 8.0