A composite slab has two layers of different materials with thermal conductivities $$k_{1}$$ and $$k_{2}$$. If each layer has same thickness, then the equivalent thermal conductivity will be:

A

$$K_{1} K_{2}$$

B

$$K_{1} + K_{2}$$

C

2$$\frac{K_{1} K_{2}}{(K_{1} + K_{2})}$$

D

$$\frac{K_{1} + K_{2}}{(K_{1} K_{2})}$$