A graph consists of the parabola $y=x^2$ and the line $y=2$. Using vertical and horizontal line tests, determine whether the parabola is a function, whether it is one-to-one, and whether its restriction x ≥ 0 is one-to-one. Provide justification.

$y=x^2$ is not a function because some vertical lines intersect it more than once; even restricting to $x\ge 0$ doesn't change that

$y=x^2$ is one-to-one because each y corresponds to a unique $x$, and the restriction $x\ge 0$ is unnecessary

$y=x^2$ is a function but not one-to-one; restricting to $x\ge 0$ yields a one-to-one function

$y=x^2$ is a function and is one-to-one only if you also restrict the range to negative numbers; otherwise, it fails all tests