Two slabs have thicknesses $d_{1}$ and $d_{2}$. Their thermal conductivities are $K_{1}$ and $K_{2}$ respectively. They are in series. The free ends of the combination of these two slabs are kept at temperatures $\theta_{1}$ and $\theta_{2}$. Assume $\theta_{1} > \theta_{2}$. The temperature $\theta$ of their common junction is

• A
  $\frac{K_{1} \theta_{1} + K_{2} \theta_{2}}{\theta_{1} + \theta_{2}}$
• B
  $\frac{K_{1} \theta_{1} d_{1} + K_{2} \theta_{2} d_{2}}{K_{1} d_{2} + K_{2} d_{1}}$
• C
  $\frac{K_{1} \theta_{1} d_{2} + K_{2} \theta_{2} d_{1}}{K_{1} d_{2} + K_{2} d_{1}}$
• D
  $\frac{K_{1} \theta_{1} + K_{2} \theta_{2}}{K_{1} + K_{2}}$