This table represents a quadratic function.

x y
1 3
3 -3
5 -5
7 -3
9 3

which statement about this function is true?

a.
the value of a is positive, so the vertex is a maximum.
b.
the value of a is negative, so the vertex is a maximum.
c.
the value of a is negative, so the vertex is a minimum.
d.
the value of a is positive, so the vertex is a minimum.