Two walls of thicknesses $d_1$ and $d_2$ and thermal conductivities $k_1$ and $k_2$ are in contact. In the steady state,if the temperatures at the outer surfaces are $T_1$ and $T_2$,the temperature at the common wall is

• A
  $\frac{k_1 T_1 d_2 + k_2 T_2 d_1}{k_1 d_2 + k_2 d_1}$
• B
  $\frac{k_1 T_1 + k_2 d_2}{d_1 + d_2}$
• C
  $\left( \frac{k_1 d_1 + k_2 d_2}{T_1 + T_2} \right) T_1 T_2$
• D
  $\frac{k_1 d_1 T_1 + k_2 d_2 T_2}{k_1 d_1 + k_2 d_2}$