Asphere of radius $R$ and made of material of relative density $\sigma$ has a concentric cavity of radius $r$. It just floats when placed in a tank full of water. The value of the ratio $R/r$ will be 

  • A

    ${\left( {\frac{\sigma }{{\sigma  - 1}}} \right)^{1/3}}$

  • B

    ${\left( {\frac{{\sigma  - 1}}{\sigma }} \right)^{1/3}}$

  • C

    ${\left( {\frac{{\sigma  + 1}}{\sigma }} \right)^{1/3}}$

  • D

    ${\left( {\frac{{\sigma  - 1}}{{\sigma  + 1}}} \right)^{1/3}}$

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  • [AIIMS 2009]