If \( \displaystyle \lim_{x \to \infty}\left(\sqrt{x^2 - x + 1} - a x\right) = b \), then the ordered pair \( (a, b) \) is:
\( (-1,\,-\frac{1}{2}) \)
\( (-1,\,\frac{1}{2}) \)
\( (1,\,\frac{1}{2}) \)
\( (1,\,-\frac{1}{2}) \)