A sphere of relative density $\sigma$ and diameter $D$ has concentric cavity of diameter $d$. The ratio of $\frac { D } { d }$, if it just floats on water in a tank is :\\
(1) $\left( \frac { \sigma - 2 } { \sigma + 2 } \right) ^ { 1 / 3 }$\\
(2) $\left( \frac { \sigma } { \sigma - 1 } \right) ^ { 1 / 3 }$\\
(3) $\left( \frac { \sigma - 1 } { \sigma } \right) ^ { 1 / 3 }$\\
(4) $\left( \frac { \sigma + 1 } { \sigma - 1 } \right) ^ { 1 / 3 }$