$A$ cylinder of radius $R$ is made of a material with thermal conductivity $K_1$. It is surrounded by a cylindrical shell of a material with thermal conductivity $K_2$,having an inner radius $R$ and an outer radius $2R$. The two ends of this composite system are maintained at two different temperatures. There is no heat loss from the surface of the cylinder in the steady state. Find the equivalent thermal conductivity of the system.

  • A
    $\frac{K_1 + 3K_2}{4}$
  • B
    $K_1 + K_2$
  • C
    $\frac{K_1 + 8K_2}{9}$
  • D
    $\frac{8K_1 + K_2}{9}$

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