The heat flow rate through parallel walls of thickness L1, L2 and L3, having surface areas A1, A2 and A3, thermal conductivities k1, k2, and k3, respectively, with the first and last walls maintained at temperatures t1 and t2 will be

• A. \(\frac{{{t_1} - {t_2}}}{{\frac{{{L_1}}}{{{A_1}{k_1}}} + \frac{{{L_2}}}{{{A_2}{k_2}}} + \frac{{{L_3}}}{{{A_3}{k_3}}}}}\)
• B. \(\frac{{{t_1} - {t_2}}}{{\frac{{{k_1}}}{{{A_1}{L_1}}} + \frac{{{k_2}}}{{{A_2}{L_2}}} + \frac{{{k_3}}}{{{A_3}{L_3}}}}}\)
• C. \(\frac{{{t_1} - {t_2}}}{{\frac{{{k_1}{A_1}}}{{{L_1}}} + \frac{{{k_2}{A_2}}}{{{L_2}}} + \frac{{{k_3}{A_3}}}{{{L_3}}}}}\)
• D. \(\frac{{{t_1} - {t_2}}}{{\frac{{{L_1}{A_1}}}{{{k_1}}} + \frac{{{L_2}}}{{{k_3}}} + \frac{{{L_3}}}{{{k_3}}}}}\)