Two radioactive materials $X_1$ and $X_2$ have decay constant $5\lambda$ and $\lambda$ respectively intially they have the saame number of nuclei, then the ratio of the number of nuclei of $X_1$ to that $X_2$ will be $\frac{1}{e}$ after a time

  • [AIPMT 2008]
  • A

    $4λ$

  • B

    $2λ$

  • C

    $\frac{1}{{2\lambda }}$

  • D

    $\;\frac{1}{{4\lambda }}$

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