A wall consists of alternating blocks of length ‘d’ and coefficient of thermal conductivity $$K_1$$ and $$K_2$$ respectively as shown in figure. The cross sectional area of the blocks are the same. The equivalent coefficient of thermal conductivity of the wall between left and right is:

A

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

B

$$\frac{2 K_1K_2}{K_1 + K_2}$$

C

$$\frac{K_1 + K_2}{3}$$

D

$$\frac{3 K_1K_2}{K_1 + K_2}$$