Two radioactive materials $X_1$ and $X_2$ have decay constants $5\lambda$ and $\lambda$ respectively. If initially they have the same number of nuclei,then the ratio of the number of nuclei of $X_1$ to that of $X_2$ will be $\frac{1}{e}$ after a time:

  • A
    $\frac{1}{4\lambda}$
  • B
    $\frac{e}{\lambda}$
  • C
    $\lambda$
  • D
    $\frac{1}{2}\lambda$

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