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