A cylindrical pipe of length \(L\) carrying hot fluid at temperature \(T_H\) is insulated by two thin concentric cylindrical layers exposed to ambient air at temperature \(T_C\). The inner insulating layer has thermal conductivity \(k_1\), thickness \(d_1\), and average radius \(r_1\). The outer insulating layer has thermal conductivity \(k_2\), thickness \(d_2\), and average radius \(r_2\). Assuming steady-state radial heat conduction where the thickness of each layer is much smaller than its radius (\(d_1 \ll r_1\) and \(d_2 \ll r_2\)), which of the following expressions represents the rate of heat transfer \(H\) through the insulation?