# The temperature of two outer surface 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 tranfer through the slab, in a steady state is $(\frac{A(T_2 - T_1) K}{x})f$, with $f$ equal to <br> <img src="http://clay6.com/mpaimg/1166_a.jpg "/>
( A ) $1$
( B ) $1/2$
( C ) $1/3$
( D ) $2/3$