A slender rod of length $L$, diameter $d$ $(L > >d)$ and thermal conductivity $k_1$ is joined with another rod of identical dimensions, but of thermal conductivity $k_2$, to form a composite cylindrical rod of length $2L$. The heat transfer in radial direction and contact resistance are negligible. The effective thermal conductivity of the composite rod is

1. $k_1+k_2 \\$
2. $\sqrt{k_1k_2} \\$
3. $\dfrac{k_1k_2}{k_1+k_2} \\$
4. $\dfrac{2k_1k_2}{k_1+k_2}$