If the load is at the surface of subgrade, the elastic strain is ________

• A. \(∆=\frac{pa}{E} 1.5\)
• B. \(∆=\frac{3pa^2}{2E} 2.5\)
• C. \(∆=\frac{a^2}{2E} 3\)
• D. \(∆=\frac{pa^2}{2E} 3.5\)