A sphere of solid material of relative density $9$ has a concentric spherical cavity and floats having just sinked in water. If the radius of the sphere be $R$, then the radius of the cavity $(r)$ will be related to $R$ as :-

- A
$r^3 = \frac{8}{9} R^3$

- B
$r^3 = \frac{2}{3} R^3$

- C
$r^3 = \frac{\sqrt 8}{3} R^3$

- D
$r^3 = \sqrt { \frac{2}{3}} R^3$

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