$A$ cylinder of thermal conductivity $k_1$ and radius $r$ is surrounded by a cylindrical shell of inner radius $r$,outer radius $2r$,and thermal conductivity $k_2$. Both cylinders have the same length and the temperature difference across their ends is the same. The equivalent thermal conductivity of the system is:

  • A
    $1/3 (k_1 + 2k_2)$
  • B
    $1/2 (2k_1 + 3k_2)$
  • C
    $1/4 (3k_1 + k_2)$
  • D
    $1/4 (k_1 + 3k_2)$

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