From an entrance exam:
If the function $f(x) = ax^2 + bx + c$ has a minimum value in the set of real numbers, what holds true?
- $a < 0$
- $a = 0$
- $a > 0$
- None of the above.
The correct answer was option 3, but wouldn't $a=b=0$ work, thus making option 4 the correct one?