Two radioactive materials $X_1$ and $X_2$ contain the same number of nuclei. If $6\lambda \, s^{-1}$ and $4\lambda \, s^{-1}$ are the decay constants of $X_1$ and $X_2$ respectively,the ratio of the number of undecayed nuclei of $X_1$ to that of $X_2$ will be $\left( \frac{1}{e} \right)$ after a time:

• A
  $\frac{1}{2\lambda} \, s$
• B
  $\frac{1}{10\lambda} \, s$
• C
  $\frac{1}{5\lambda} \, s$
• D
  $\frac{1}{\lambda} \, s$