The half-life of a radioactive isotope $X$ is $1.37 \times 10^9 \, y$. It decays to a stable isotope $Y$. In a rock sample from the moon,the ratio of $X$ to $Y$ is $1:7$. The age of the rock is:

  • A
    $1.96 \times 10^8 \, y$
  • B
    $3.85 \times 10^9 \, y$
  • C
    $4.11 \times 10^9 \, y$
  • D
    $9.06 \times 10^9 \, y$

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Define the decay constant and state its $SI$ unit.

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