The stress σz at any point (r,z) below the pavement is __________

• A. \(σ_z=\frac{3P}{2πz^2}\left[\frac{1}{1+(\frac{r}{z})^3}\right]^\frac{5}{2}\)
• B. \(σ_z=\frac{3P}{2πz^2}\left[\frac{1}{(\frac{r}{z})^2}\right]^\frac{5}{2}\)
• C. \(σ_z=\left[\frac{1}{1+(\frac{r}{z})^2}\right]^\frac{5}{2}\)
• D. \(σ_z=\frac{3P}{2πz^2}\left[\frac{1}{1+(\frac{r}{z})^2}\right]^\frac{5}{2}\)