Which of the following is true for elastic potential energy density?

• A
  Energy density $=$ $\frac{1}{2} \times \text{strain} \times \text{stress}$
• B
  Energy density $=$ $(\text{strain})^2 \times \text{volume}$
• C
  Energy density $=$ $\text{strain} \times \text{volume}$
• D
  Energy density $=$ $\text{stress} \times \text{volume}$