If δ is the angle of surface friction, then frictional resistance Fᵢ is ___________

• A. Fᵢ=γzᵢ tanδ*surface area of reinforcement
• B. Fᵢ=γzᵢ cosδ
• C. Fᵢ=γzᵢ sinδ*surface area of reinforcement
• D. Fᵢ=γzᵢ cotδ*surface area of reinforcement