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 temperaturesat the outer surfaces are $$T_1$$ and $$T_2$$ the temperature at the common wall is

A

$$\frac{(k_1 T_1d_2 + k_2T_2d_1)}{(k_1d_2 + k_2d_1)}$$

B

$$\frac{(k_1 T_1 + k_2d_2)}{(d_1 + d_2)}$$

C

$$\left[\frac{(k_1d_1 + k_2d_2)}{(T_1 + T_2)}\right]^*T_1T_2$$

D

$$\frac{(k_1d_1T_1 + k_2d_2T_2)}{(k_1d_1 + k_2d_2)}$$