Two radioactive substances $X$ and $Y$ initially contain equal number of nuclei. $X$ has a half-life of $1\, hour$ and $Y$ has a half-life of $2\, hours$. After $2\, hours$,the ratio of the activity of $X$ to the activity of $Y$ is

  • A
    $1 : 4$
  • B
    $1 : 2$
  • C
    $1 : 1$
  • D
    $2 : 1$

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