Two rods A and B of different materials are welded together as shown in figure. Their thermal conductivities are $$K_1 and K_2.$$ The thermal conductivity of the composite rod will be

A

$$K_1 + K_2$$

B

$$2(K_1 + K_2)$$

C

$$\frac{K_1 + K_2}{2}$$

D

$$\frac{3(K_1 + K_2)}{2}$$