Which of the following is the correct expression and value for the heat current (in watts) through the compound bar formed by soldering an iron bar (L1 = 0.1 m, A1 = 0.02 m ² , K1 = 79 W m⁻¹ K⁻¹) and a brass bar (L2 = 0.1 m, A2 = 0.02 m ² , K2 = 109 W m⁻¹ K⁻¹) together, with the free ends of the iron bar and brass bar maintained at 373 K and 273 K, respectively?