Two square slabs of different materials, each having cross-sectional area \(A\) and thickness \(L\), are joined face-to-face in series. The outer face of the first slab with thermal conductivity \(k_1\) is held at a constant high temperature \(T_H\), while the outer face of the second slab with thermal conductivity \(k_2\) is held at a constant low temperature \(T_C\). Assuming heat flows only perpendicular to the faces and the system has reached a steady state, which of the following expressions represents the rate of heat transfer \(\dfrac{Q}{\Delta t}\) through the combined slabs?