Bluebook试题2 · 第 22 题

f(x) = ax² + 4x + c
In the given quadratic function, a and c are constants. The graph of y = f(x) in the xy-plane is a parabola that opens upward and has a vertex at the point (h, k), where h and k are constants.If k < 0 and f(-9)= f(3), which of the following must be true?
I.c<0
II. a≥1