An accident in a nuclear laboratory resulted in deposition of a certain amount of radioactive material of half-life $18$ days inside the laboratory. Tests revealed that the radiation was $64$ times more than the permissible level required for safe operation of the laboratory. What is the minimum number of days after which the laboratory can be considered safe for use?

  • [IIT 2016]
  • A

    $64$ 

  • B

    $90$

  • C

    $108$

  • D

    $120$

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