Determine the boundary of the domain of the function f(x,y)=9−x2−y2f(x,y) = \sqrt{9 - x^2 - y^2}f(x,y)=9−x2−y2 in the xyxyxy-plane.
A circle of radius 3 centered at the origin
A parabola opening downwards
A straight line y=9−xy = 9 - xy=9−x
A sphere of radius 3 centered at the origin