A radioactive isotope sample has a decay constant $\lambda= 2.31 \times 10^{-4} \text{year}^{-1}$. Initially the sample contains $8000 \times 10^{15}$ atoms. The sample decays to $25 \%$ of its initial value. The number of years it takes to decay to this stage is closest to ________.
- $3000$
- $6000$
- $900$
- $2600$
