The following equation is that of a parabola. Using its vertex and the direction in which it opens, solve for its domain and range, and tell if the given relation is a function or not.
y = - x² + 10x - 21

• A. Domain: (- ∞, 5), Range: (- ∞, ∞): Function
• B. Domain: (- ∞, 5), Range: (- ∞, ∞): Not a function
• C. Domain: (- ∞, ∞), Range: (- ∞, 4]: Function
• D. Domain: (- ∞, ∞), Range: (- ∞, 4]: Not a function

Answer: (C)