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 $\frac{1}{e}$ ?

Question

( A ) $\frac{1}{9 \lambda}$
( B ) $\frac{1}{ \lambda}$
( C ) $\frac{1}{8 \lambda}$
( D ) $\frac{1}{7 \lambda}$