Two thin metallic spherical shells of radii $r_{1}$ and $r_{2}$ $(r_{1} < r_{2})$ are placed with their centres coinciding. $A$ material of thermal conductivity $K$ is filled in the space between the shells. The inner shell is maintained at temperature $\theta_{1}$ and the outer shell at temperature $\theta_{2}$ $(\theta_{1} < \theta_{2})$. The rate at which heat flows radially through the material is:

• A
  $\frac{4 \pi K r_{1} r_{2}(\theta_{2}-\theta_{1})}{r_{2}-r_{1}}$
• B
  $\frac{\pi r_{1} r_{2}(\theta_{2}-\theta_{1})}{r_{2}-r_{1}}$
• C
  $\frac{K(\theta_{2}-\theta_{1})}{r_{2}-r_{1}}$
• D
  $\frac{K(\theta_{2}-\theta_{1})(r_{2}-r_{1})}{4 \pi r_{1} r_{2}}$