I can't speak to the cactus specifically, but any sphere has a surface area of

and a volume of

, so the area-to-volume ratio wold be


Where <em>r</em> is the radius of the sphere.
I believe you graph this.. please let me know if wrong and I’ll help solve otherwise! The picture is the answer on how to graph
First you subtract 3 from both sides. 3-3 and 7-3, that cancels out 3 which leaves you with r>4.
Answer: its the first one "opposite sides with equal length"