In a composite rod, when two rods of different lengths and of the same area of cross-section are joined end to end, then if K is the effective coefficient of thermal conductivity, $\frac{{\mathrm{I}}_1+{\mathrm{I}}_2}{\mathrm{K}}\text{ is equal to }$

• A

  $\frac{{\mathrm{I}}_1}{{\mathrm{K}}_1}-\frac{{\mathrm{I}}_2}{{\mathrm{K}}_2}$

• B

  $\frac{{\mathrm{I}}_1}{{\mathrm{K}}_2}-\frac{{\mathrm{I}}_2}{{\mathrm{K}}_1}$

• C

  $\frac{{\mathrm{I}}_1}{{\mathrm{K}}_1}+\frac{{\mathrm{I}}_2}{{\mathrm{K}}_2}$

• D

  $\frac{{\mathrm{I}}_1}{{\mathrm{K}}_2}\times \frac{{\mathrm{I}}_2}{{\mathrm{K}}_1}$