Two radioactive materials $A$ and $B$ having decay constants $7 \lambda$ and $\lambda$ respectively,initially have the same number of nuclei. The time taken for the ratio of the number of nuclei of material $B$ to that of $A$ to be $e$ is:

  • A
    $\frac{1}{\lambda}$
  • B
    $\frac{1}{6 \lambda}$
  • C
    $\frac{1}{7 \lambda}$
  • D
    $\frac{1}{8 \lambda}$

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