If the inner and outer walls of a hollow sphere having surface areas of $$A_1$$ and $$A_2$$ and inner and outer radii $$r_1$$ and $$r_2$$ are Maintained at temperatures $$t_1$$ and $$t_2$$, then rate of heat flow will be

A

$$\frac{k}{\sqrt{A_1A_2}} \frac{t_1 - t_2}{r_1 - r_2}$$

B

$$k\sqrt{A_1A_2} \frac{t_1 - t_2}{r_2 - r_1}$$

C

$$4 \pi k \frac{t_1 - t_2}{\sqrt{A_1A_2}}$$

D

$$4 \pi kr_1r_2 \frac{t_1 - t_2}{\sqrt{A_1A_2}}$$