Cho \(f\left( x \right) = \left( {x - 1} \right)\left( {2x + 1} \right).\)Tìm mệnh đề đúng:

A. \(f\left( x \right) \le 0 \Leftrightarrow  - \frac{1}{2} \le x \le 1\)

B. \(f\left( x \right) \ge 0 \Leftrightarrow x \le  - \frac{1}{2}\)

C. \(f\left( x \right) > 0 \Leftrightarrow x > 1\)

D. \(f\left( x \right) < 0 \Leftrightarrow x < 1\)