Two slabs A and B of different materials but of the same thickness are joined and to end to form a composite slab. The thermal conductivities of A and B are $'k_1'$ and $'k_2$ respectively. A steady temperature distance of $12^{\circ}$ is maintained across the composite slab. If $k_1=\large\frac{k_2}{2}$, the temperature difference across slab A is

Question

$$\begin {array} {1 1} (a)\;4^{\circ}C & \quad (b)\;6^{\circ}C \\ (c)\;8^{\circ}C & \quad (d)\;10^{\circ}C \end {array}$$