A composite slab has twolayers of different materials with thermal conductivities $$k_1$$ and $$k_2$$. If each layer has the same thickness , then the equivalent thermal conductivity of the slab will be

A

$$\frac{2 k_1 k_2}{k_1 + k_2}$$

B

$$\frac{k_1 + k_2}{k_1 k_2}$$

C

$$k_1 k_2$$

D

$$k_1 + k_2$$