Two rods A and B of different materials are welded together as shown in figure. Their thermal conductivities are $K_1$ and $K_2$. The thermal conductivity of the composite rod will be

Question

$\begin{array}{1 1} (1)\large\frac{K_1+K_2}{2} \\ (2) \large\frac{3(K_1+K_2)}{2} \\ (3) K_1+K_2 \\ (4) 2(K_1+K_2) \end{array}$