Two radioactive substances A and B have decay constants $5 \lambda$ and $\lambda $ respectively. At $t=0$ they have the same number of nuclei. The ratio of number of nuclei of A to those of B will be $(\frac{1}{e} )^2$ after a time interval :-

Question

( A ) $\frac{1}{2 \lambda}$
( B ) $4 \lambda$
( C ) $2 \lambda$
( D ) $\frac{1}{4 \lambda}$