Based on the measured strain, the axial elongation of rod $\left(1\right)$ must be $1.8 \mathrm{mm}$. Since rigid bar $\mathrm{ABC}$ is attached to rod $\left(1\right)$, the rigid bar must deflect the same amount.

The symmetry of the assembly ensures that the rigid bar will remain horizontal as it deflects. Therefore, the deflection of the rigid bar at joints $\mathrm{A}$ and $\mathrm{C}$ must be the same as the deflection at $\mathrm{B}$.