A solid sphere of radius $r_1 = 20$ $mm$ is placed concentrically inside a hollow sphere of radius $r_2 = 30$ $mm$ as shown in the figure.

The view factor $F_{21}$ for radiation heat transfer is

- $\dfrac{2}{3} \\$
- $\dfrac{4}{9} \\$
- $\dfrac{8}{27} \\$
- $\dfrac{9}{4}$