$A$ wall consists of alternating blocks of length $d$ and coefficients of thermal conductivity $K_{1}$ and $K_{2}$ respectively,as shown in the figure. The cross-sectional areas of the blocks are the same. The equivalent coefficient of thermal conductivity of the wall between the left and right sides is

• A
  $\frac{2K_{1}K_{2}}{K_{1} + K_{2}}$
• B
  $\frac{K_{1} + K_{2}}{3}$
• C
  $\frac{K_{1}K_{2}}{2(K_{1} + K_{2})}$
• D
  $\frac{K_{1} + K_{2}}{2}$