Lilly Ann went to the pet store with her mother to buy a gerbil. After choosing her gerbil, she and her mother went to look at cages and found that one of the brands makes several self-contained cage pieces that can be combined or interchanged by snapping them together. Lilly Ann chose a sleep component with a built-in hut, a play component with a built-in wheel, a food component with a built-in food dish and water bottle, and several tubes to connect all the components together. This type of cage is an example of ______.