The radioactivity of a given sample of whisky due to tritium (half-life $12.5 \text{ years}$) was found to be only $3\%$ of that measured in a recently purchased bottle marked "$7 \text{ years}$ old". The sample must have been prepared about:

  • A
    $220 \text{ years}$ back
  • B
    $300 \text{ years}$ back
  • C
    $400 \text{ years}$ back
  • D
    $70 \text{ years}$ back

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