$A$ radioactive substance has a half-life of $10^8$ years and an activity of $10^4 \,Bq$. The number of atoms of this substance present is

  • A
    $9.1 \times 10^{19}$
  • B
    $6.7 \times 10^9$
  • C
    $4.5 \times 10^{19}$
  • D
    $5 \times 10^{20}$

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