Two metal rods have their thermal conductivities $\;K_{1}\;$ and $\;K_{2}\;$ in the ratio 3 : 2 so that their thermal resistance are the same . Then their lengths are is the ratio ( Area of C .S . are the same )

Question

$(a)\;9 : 4\qquad(b)\;3 : 2\qquad(c)\;2 : 3\qquad(d)\;1 : 1$