The figure shows three different arrangements of materials $1, 2$,and $3$ to form a wall. The thermal conductivities are $k_1 > k_2 > k_3$. 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
  $\Delta T_a > \Delta T_b > \Delta T_c$
• B
  $\Delta T_a = \Delta T_b = \Delta T_c$
• C
  $\Delta T_a = \Delta T_b > \Delta T_c$
• D
  $\Delta T_a = \Delta T_b < \Delta T_c$