If the graph of \( f ( x ) = a x ^ { 2 } + b x + c , a \neq 0 , \) has a maximum value at its vertex, which of the following conditions must be true? Select all that apply.
\( \square \) A. \( a < 0 \) B. \( - \frac { b } { 2 a } < 0 \)
\( \square \) C. \( - \frac { b } { 2 a } > 0 \)
D. \( a > 0\)