Determine the vertical total stress, pore pressure, and vertical effective stress at the mid-depths of the three soil layers; i.e., at depths (z) of 45.0 ft, 106.5 ft, and 181.5 ft.

A. Use the equation σ_v = γz for total stress.
B. Pore pressure is independent of depth.
C. Calculate effective stress using the formula σ_v' = σ_v - Pore pressure.
D. Total stress is only relevant in the topsoil layer.