A sphere of radius R has a concentric cavity o{ radius r. The relative density of the material of the sphere is o. It just floats when placed in a tank full of water. The ratio R/r is

• A

  $[\sigma /(\sigma -1){]}^{1/3}$

• B

  $\left[\right(\mathrm{\sigma }-1)/\mathrm{\sigma }{]}^{1/3}$

• C

  $[\mathrm{\sigma }/(\mathrm{\sigma }-1){]}^{1/2}$

• D

  $\left[\right(\mathrm{\sigma }-1)/\mathrm{\sigma }{]}^{1/2}$