The vertex of a parabola is in the first quadrant of a coordinate grid. A line with a negative slope passes through the origin. If the parabola and line intersect at the origin, which statement must be true?
a. The parabola opens downward.
b. The parabola opens upward.
c. The slope of the line is equal to –1.
d. The slope of the line is not equal to –1.