The temperature of two outer surfaces of a composite slab, consisting of two materials having coefficients of thermal conductivity $K$ and $2K$ and thickness $x$ and $4x$ respectively, are $T_2$ and $T_1$ ($T_2>T_1$). The rate of heat transfer through the slab in a steady state is $\frac{A(T_2-T_1)K}{x}f$, where $f$ is:

1. A
   $1$
2. B
   $1/2$
3. C
   $2/3$
4. D
   $1/3$