The figure shows three different series arrangements of materials $1$, $2$ and $3$ to form a wall, each layer having equal thickness. Thermal conductivities satisfy $k_1 > k_2 > k_3$. The left side of the wall is $20\,^\circ\text{C}$ higher than the right side. The temperature difference $\Delta T$ across material $1$ has the following relation in the three cases: (a) layers $1$–$2$–$3$, (b) layers $1$–$3$–$2$, and (c) layers $2$–$3$–$1$.