In a composite rod,when two rods of different lengths and of the same area are joined end to end,then if $K$ is the effective coefficient of thermal conductivity,$\frac{{\ell _1} + {\ell _2}}{K}$ is equal to
- A$\frac{{\ell _1}}{{{K_1}}} - \frac{{\ell _2}}{{{K_2}}}$
- B$\frac{{\ell _1}}{{{K_2}}} - \frac{{\ell _2}}{{{K_1}}}$
- C$\frac{{\ell _1}}{{{K_1}}} + \frac{{\ell _2}}{{{K_2}}}$
- D$\frac{{\ell _1}}{{{K_2}}} + \frac{{\ell _2}}{{{K_1}}}$
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