An aluminium rod with Young’s modulus \( Y = 7.0 \times 10^{10} \, \text{N}/\text{m}^{2} \) undergoes elastic strain of \( 0.04\% \). The energy per unit volume stored in the rod in SI unit is:
\( 5600 \)
\( 2800 \)
\( 11200 \)
\( 8400 \)