The heat flow rate through parallel walls of thickness $$L_1, L_2$$ and $$L_3$$, having surface areas $$A_1, A_2$$ and $$A_3$$, thermal conductivities $$k_1, k_2$$ and $$k_3$$,
respectively, with the first and last walls maintained at temperatures $$t_1$$ and $$t_2$$ will be

A

$$\frac{t_1 - t_2}{\frac{L_1}{A_1k_1} + \frac{L_2}{A_2k_2} + \frac{L_3}{A_3k_3}}$$

B

$$\frac{t_1 - t_2}{\frac{k_1}{A_1L_1} + \frac{k_2}{A_2L_2} + \frac{k_3}{A_3L_3}}$$

C

$$\frac{t_1 - t_2}{\frac{k_1A_1}{L_1} + \frac{k_2A_2}{L_2} + \frac{k_3A_3}{L_3}}$$

D

$$\frac{t_1 - t_2}{\frac{L_1A_1}{k_1} + \frac{L_2}{k_2} + \frac{L_3}{k_3}}$$