If p₁ and p₂ are effective lateral loadings at the bottom and top exerted by a level earth subjected to a super-load on the vertical face of height h of a retaining wall, the horizontal pressure p per unit length of the wall, is

• A. [(p₁ - p₂)/2] h
• B. [(p₁ + p₂)/4] h
• C. [(p₁ + p₂)/2] h
• D. (p₁ - p₂) ⅔h