Properties of Solids and Liquids: JEE Main 2021 Physics question

Two thin metallic spherical shells of radii r₁ and r₂ (r₁ < r₂) 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 :-