Equivalent thermal conductivity of slabs or rods joined in series

1. A.
   $\frac{2{\text{ }K}_{1}{\text{ }K}_{2}}{{\text{ }K}_{1} + K_{2}}$
2. B.
   $\frac{K_{1} + K_{2}}{2{\text{ }K}_{1}{\text{ }K}_{2}}$
3. C.
   $\frac{K_{1} + K_{2}}{{\text{ }K}_{1}{\text{ }K}_{2}}$
4. D.
   $\frac{K_{1}{\text{ }K}_{2}}{{\text{ }K}_{1} + K_{2}}$

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