The ratio of thermal conductivity of two rods of different material is $5: 4.$ The two rods of same area of crosssection and same thermal resistance will have the length in the ratio

Five rods of same dimensions are arranged as shown in the figure. They have thermal conductivities $K_1,K_2$. $K_3,K_4$ and $K_5$. When points $A$ and $C$ are maintained at different temperature, no heat flows through the central rod if