A scientist found a specimen in which the decay constant of nuclei X and nuclei Y are $9 \lambda$ and $\lambda$ respectively. At $t=0 \mathrm{~s}$ the number of X and Y nuclei were the same. The ratio of number of X nuclei to that of Y nuclei will reduce to $(1 / e)^4$ after a time interval of

• A

  $1 / 4 \lambda$
• B

  $1 / 3 \lambda$
• Correct answer
  C

  $1 / 2 \lambda$
• D

  $1 / \lambda$

Hence, the correct answer is option: 'C'.