Bluebook试题2 · 第 13 题

The function \( f\left(x\right)\ =\ \frac{1}{9}\left(x-7\right)^2+3 \)  gives a metal ball's height above the ground f(x), in inches, x seconds after it started moving on a track, where 0 ≤ x ≤10. Which of the following is the best interpretation of the vertex of the graph of y = f(x) in the xy-plane?