The activity of a sample is $ 64 \times 10^{-5}\ Ci$ . Its half-life $3$ days. The activity will become$5 \times {10^{ - 6}}\ Ci$ after ........... $days$

  • A

    $12$

  • B

    $ 7$

  • C

    $18 $

  • D

    $21$

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