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

86-78

  • [NEET 2017]
  • A

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

  • B

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

  • C

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

  • D

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

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  • [AIIMS 2017]