$A$ sample of a radioactive element contains $8 \times 10^{16}$ active nuclei. The half-life of the element is $15 \text{ days}$. The number of nuclei decayed after $60 \text{ days}$ is:

  • A
    $7.5 \times 10^{16}$
  • B
    $2.0 \times 10^{16}$
  • C
    $0.5 \times 10^{16}$
  • D
    $4.0 \times 10^{16}$

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