Two radioactive substances $A$ and $B$ have decay constants $5\lambda$ and $\lambda$ respectively. At $t = 0$,a sample has the same number of the two nuclei. The time taken for the ratio of the number of nuclei to become $(1/e)^2$ will be
- A$1/\lambda$
- B$1/4\lambda$
- C$2/\lambda$
- D$1/2\lambda$
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