A concrete sphere of radius $R$  has a cavity of radius $ r$  which is packed with sawdust. The specific gravities of concrete and sawdust are respectively $2.4$  and $0.3$  for this sphere to float with its entire volume submerged under water. Ratio of mass of concrete to mass of sawdust will be

  • [AIIMS 1995]
  • A

    $8$

  • B

    $4$

  • C

    $3$

  • D

    $0$

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