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.