Cells, tree, digestive system, public transportation system, government systems, school systems, families, dog, forest, pond, prairie, communication systems, towns, insect, solar system, weather systems, bird, correctional system, squirrel, soil, etc.
It will be better for the population
All of these are necessary, but what you need the most I would say is
D. Movement
Without movement, your body can stiff up, causing muscle problems which can lead to many various things.
