Equivalent thermal conductivity of slab having two wall of different material.

Question

 A composite wall consist of two layers of different materials having conductivities k₁ and k₂. For equal thickness of the two layers, the equivalent thermal conductivity of the slab will be                         (IES-97)

($k_1+k_2$                $k_1*k_2$

_{(c)$2\frac{k_1k_2}{k_1+k_2}$   (d)$\frac{k_1+k_2}{k_1k_2}$}