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 $$\frac{1}{e}$$ ?

A

$$\frac{1}{8\lambda}$$

B

$$\frac{1}{9\lambda}$$

C

$$\frac{1}{\lambda}$$

D

$$\frac{1}{7\lambda}$$