Two radioactive materials $X_1$ and $X_2$ have decay constants $5 \lambda$ and $\lambda$ respectively. Initially,they have the same number of nuclei. After time $t$,the ratio of the number of nuclei of $X_1$ to that of $X_2$ is $\frac{1}{e}$. Then $t$ is equal to:
- A$\frac{\lambda}{2}$
- B$\frac{e}{\lambda}$
- C$\lambda$
- D$\frac{1}{4 \lambda}$
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