Stress and strain — practice question

For an elastic material with Young modulus $E$, a tensile stress $S$ is applied. Hooke’s Law holds. What expression gives the elastic energy stored per unit volume of the material?

• A$\frac{S^{2}}{2E}$
• B$\frac{S^{2}}{E}$
• C$\frac{E}{2S^{2}}$
• D$\frac{2E}{S^{2}}$