A section of Earth's crust consists of two flat, parallel rock layers of equal cross-sectional area \(A\). Layer 1 (bottom layer) has thickness \(L_1\) and thermal conductivity \(k_1\). Layer 2 (top layer) has thickness \(L_2\) and thermal conductivity \(k_2\). The bottom surface of Layer 1 is maintained at temperature \(T_H\) by geothermal heat, while the top surface of Layer 2 is at temperature \(T_C\), where \(T_H > T_C\). Assuming heat flows at a steady rate perpendicularly through the layers with no heat transfer through the sides, which of the following expressions represents the temperature \(T_I\) at the interface between the two rock layers?