$A$ parent nucleus $X$ decays into a daughter nucleus $Y$,which in turn decays into $Z$. The half-lives of $X$ and $Y$ are $40000 \, yr$ and $20 \, yr$,respectively. In a certain sample,it is found that the number of $Y$ nuclei hardly changes with time. If the number of $X$ nuclei in the sample is $4 \times 10^{20}$,the number of $Y$ nuclei present in it is

  • A
    $2 \times 10^{17}$
  • B
    $2 \times 10^{20}$
  • C
    $4 \times 10^{23}$
  • D
    $4 \times 10^{20}$

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