Cho f( x ) = a2 + bx + c ,(a # 0 ). Điều kiện để \(f (x)\le 0 , \forall x \in R\) là

A. \(\left\{ \begin{array}{l} a > 0\\ \Delta \le 0 \end{array} \right.\)

B. \(\left\{ \begin{array}{l} a > 0\\ \Delta \ge 0 \end{array} \right.\)

C. \(\left\{ \begin{array}{l} a > 0\\ \Delta < 0 \end{array} \right.\)

D. \(\left\{ \begin{array}{l} a < 0\\ \Delta > 0 \end{array} \right.\)