$A$ freshly prepared radioactive source of half-life $2\, hours$ emits radiation of intensity which is $64\, \times$ the permissible safe level. The minimum time after which it would be possible to work safely with the source is .......... $hours$.

  • A
    $6$
  • B
    $12$
  • C
    $24$
  • D
    $128$

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