Consider two insulating sheets with thermal resistances $R_1$ and $R_2$ as shown in the figure. The temperature $\theta$ at the junction is:

• A
  $\frac{{\theta _1}{\theta _2}{R_1}{R_2}}{{({R_1} + {R_2})({\theta _1} + {\theta _2})}}$
• B
  $\frac{{\theta _1}{R_1} + {\theta _2}{R_2}}{{{R_1} + {R_2}}}$
• C
  $\frac{{({\theta _1} + {\theta _2})\left( {{R_1}{R_2}} \right)}}{{R_1^2 + R_2^2}}$
• D
  $\frac{{\theta _1}{R_2} + {\theta _2}{R_1}}{{{R_1} + {R_2}}}$