Two thin metallic spherical shells of radii \(r_1\) and \(r_2\) \((r_1 < r_2)\) are placed with their centres coinciding. A thermal conductivity material, \(K\), fills the space between the shells. The inner shell is maintained at a 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 \(\dfrac{dQ}{dt}\), is: