Two radioactive materials $X_1$ and $X_2$ have decay constants $5\lambda$ and $\lambda$ respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of $X_1$ to - that of $X_2$ will be $\frac{1}{e}$ after a time

Question

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