The temperature $\theta$ at the junction of two insulating sheets,having thermal resistances $R_{1}$ and $R_{2}$ as well as top and bottom temperatures $\theta_{1}$ and $\theta_{2}$ (as shown in figure) is given by

• A
  $\frac{\theta_{1} R_{1}+\theta_{2} R_{2}}{R_{1}+R_{2}}$
• B
  $\frac{\theta_{1} R_{2}-\theta_{2} R_{1}}{R_{2}-R_{1}}$
• C
  $\frac{\theta_{1} R_{2}+\theta_{2} R_{1}}{R_{1}+R_{2}}$
• D
  $\frac{\theta_{2} R_{1}+\theta_{1} R_{2}}{R_{1}+R_{2}}$