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Example $3$: The graph of the function $y=f(x)$ in the $x y$-plane always has a negative rate of change. Which of the following definitions for the variables $x$ and $y$ would best model the function $f$ ?$\newline$(A) $x=$ the age, in years, of a young child; $y=$ the height, in inches, of the young child$\newline$(B) $x=$ the total number of points scored in a basketball game; $y=$ the time remaining, in seconds, in the game.$\newline$(C) $x=$ the time, in seconds, since a ball was thrown straight up in the air; $y=$ the height, in feet, of the ball$\newline$(D) $x=$ the radius, in meters, of a circle; $y=$ the area, in square inches, of the circle