The temperature of the two outer surface of a composite slab consisting of two materials having cocflicient of thermal conductivities $K$ and $2 K$, thickness $x$ and $4 x$ respectively are $T_{3}$ and $T_{1}\left(T_{2}>T_{1}\right) .$ The rate of heat transfer through the slab, in a steady state is $F\left[\frac{A\left(T_{2}-T_{1}\right) K}{x}\right]$, the value of $F$ is
(a) 1
(b) $1 / 2$
(c) $2 / 3$
(d) $1 / 3$