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
  $\frac{{\ell _1}}{{{K_2}}} + \frac{{\ell _2}}{{{K_1}}}$