Two different soil types (Soil 1 and Soil 2) are used as backfill behind a retaining wall as shown in the figure, where γt is total unit weight, and c' and ϕ' are effective cohesion and effective angle ofshearing resistance. The resultant active earth force per unit length (in kN/m) acting on the wall is:

• A. 31.7
• B. 35.2
• C. 51.8
• D. 57