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

Options

A. `(K(theta_(2)-theta_(1))(r_(2)-r_(1)))/(4pir_(1)r_(2))`
B. `(K(theta_(2)-theta_(1)))/(r_(2)-r_(1))`
C. `(pir_(1)r_(2)(theta_(2)-theta_(1)))/(r_(2)-r_(1))`
D. `(4piKr_(1)r_(2)(theta_(2)-theta_(1)))/(r_(2)-r_(1))`