Radioactive material 'A' has decay constant '$8\lambda$ ' and material 'B' has decay constant '$\lambda$ '. Initially they have same number of nuclei. After what time, the ratio of number of nuclei of material 'B' to that 'A' will be $e$?

1. A
   $\frac{1}{7\lambda }$
2. B
   $\frac{1}{8\lambda }$
3. C
   $\frac{1}{9\lambda }$
4. D
   $\frac{1}{\lambda }$