It's wheels, it's always gonna be wheels, most things will have 3 to 4 wheels, trucks have 2 doors 4 to 8 wheels, then you have the trailer that usually has 8 wheels and 2 doors, then you have any industrial machinery that pulls something throughit, because rollers are wheels, also, now I know people will say that houses have lots of doors, but think about it, your vacume has wheels/no door, your fridge has 4 wheels 2 doors, no matter how you try to play this game, wheels always come out on top, they're infinitely more important to modern society than doors are.