A prismatic linearly elastic bar of length $L$, cross-sectional area $A$, and made up of a material with Young’s modulus $E$, is subjected to axial tensile force as shown in the figures. When the bar is subjected to axial tensile forces $P_1$ and $P_2$, the strain energies stored in the bar are $U_1$ and $U_2$, respectively.

If $U$ is the strain energy stored in the same bar when subjected to an axial tensile force $(P_1 + P_2),$ the correct relationship is

1. $U = U_1 + U_2$
2. $U = U_1 - U_2$
3. $U < U_1 + U_2$
4. $U > U_1 + U_2$