A slab consists of two layers of different materials of the same thickness and having thermal conductivities `K_(1)` and `K_(2)`. The equivalent thermal conductivity of the slab is
A. `K_(1) + K_(2)`
B. `(K_(1) K_(2))/(K_(1) + K_(2))`
C. `sqrt(K_(1) K_(2))`
D. `(2K_(1) K_(2))/(K_(1) + K_(2))`