Three identical metal rods `A`, `B` and `C` are placed end to end and a temperature difference is maintained between the free ends of `A` and `C`. If the thermal conductivity of `B(K_(B))` is twice that of `C(K_(C ))` and half that of `A(K_(A))`, `(K_(A)=49w//mK)` calculate the effective thermal conductivity of the system ?

A. `1/3 K_(A)`

B. `3K_(A)`

C. `2K_(A)`

D. `2/3 K_(A)`