The heat flow rate through parallel walls of thickness $$L_1, L_2, and L_3$$ and having surface areas $$A_1, A_2, and A_3$$, thermal conductivities $$k_1, k_2, and k_3,$$ respectively and 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

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