Two radioactive materials $M_1$ and $M_2$ have decay constants $9 \lambda$ and $\lambda$ respectively. Initially,they have the same number of nuclei. The ratio of the number of nuclei of $M_1$ to that of $M_2$ will be $\frac{1}{e}$ after a time interval of:

  • A
    $\frac{9}{10 \lambda}$
  • B
    $\frac{1}{10 \lambda}$
  • C
    $\frac{1}{9 \lambda}$
  • D
    $\frac{1}{8 \lambda}$

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