At any instant,two elements $X_1$ and $X_2$ have the same number of radioactive atoms. If the decay constants of $X_1$ and $X_2$ are $10\lambda$ and $\lambda$ respectively,then the time when the ratio of their atoms becomes $\frac{1}{e}$ will be:

  • A
    $\frac{1}{5\lambda}$
  • B
    $\frac{1}{11\lambda}$
  • C
    $\frac{1}{6\lambda}$
  • D
    $\frac{1}{9\lambda}$

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In a mean life of a radioactive sample,

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