In a composite rod,two rods of different lengths and the same cross-sectional area are joined end to end. If $K$ is the equivalent coefficient of thermal conductivity of the composite rod,then $\left( \frac{\ell_1 + \ell_2}{K} \right)$ 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
  $\frac{\ell_1}{K_2} + \frac{\ell_2}{K_1}$