$($B$)$ Two walls of same thickness and cross sectional area have thermal conductivities in the ratio $1$:$2.$ If same temperature difference is maintained across the two faces of both the walls, what is the ratio of heat flow $\frac{Q_1}{Q_2}?($given that the Fourier's law of heat conduction as $Q^{}=-kA(\frac{dT}{dx})$ where the symbols have the usual meaning.$)$

$[3]$