Two materials having coefficients of thermal conductivity $3K$ and $K$ and thickness $d$ and $3d$,respectively,are joined to form a slab as shown in the figure. The temperatures of the outer surfaces are $\theta_2$ and $\theta_1$ respectively $(\theta_2 > \theta_1)$. The temperature at the interface is

• A
  $\frac{\theta_2 + \theta_1}{2}$
• B
  $\frac{\theta_1}{10} + \frac{9\theta_2}{10}$
• C
  $\frac{\theta_1}{3} + \frac{2\theta_2}{3}$
• D
  $\frac{\theta_1}{6} + \frac{5\theta_2}{6}$