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) \(k \sqrt{A_{1} A_{2}} \cdot \frac{t_{1}-t_{2}}{r_{2}-r_{1}}\)

(b) \(\frac{k}{\sqrt{A_{1} A_{2}}} \cdot \frac{t_{1}-t_{2}}{r_{2}-r_{1}}\)

(c) \(\frac{4 \pi k}{\sqrt{A_{1} A_{2}}} \cdot \frac{t_{1}-t_{2}}{r_{2}-r_{1}}\)

(d) \(\frac{4 \pi k r_{1} r_{2}}{\sqrt{A_{1} A_{2}}} \cdot \frac{t_{1}-t_{2}}{r_{2}-r_{1}}\)