Two radioactive materials $X_1$ and $X_2$ contain the same number of nuclei. If $6\lambda \, s^{-1}$ and $4\lambda \, s^{-1}$ are the decay constants of $X_1$ and $X_2$ respectively,the ratio of the number of undecayed nuclei of $X_1$ to that of $X_2$ will be $\left( \frac{1}{e} \right)$ after a time:

  • A
    $\frac{1}{2\lambda} \, s$
  • B
    $\frac{1}{10\lambda} \, s$
  • C
    $\frac{1}{5\lambda} \, s$
  • D
    $\frac{1}{\lambda} \, s$

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