The heat flow equation through a sphere of inner radius \(r_{1}\) and outer radius \(r_{2}\) is to be written in the same form as that for heat flow through a plane wall. For wall thickness \(\left(r_{2}-r_{1}\right)\), the equivalent mean radius for the spherical shell is:

(a) \(\frac{r_{1}+r_{2}}{2}\)

(b) \(r_{1} r_{2}\)

(c) \(\sqrt{r_{1} r_{2}}\)

(d) \(\frac{r_{1}+r_{2}}{\log _{e}\left(r_{2} / r\right)}\)