Answer: the order from least to most complex (smallest to largest?): cell, tissue, organ, organ system, organism.
Explanation: Cells make up tissue, tissue like muscle tissue makes up organs, organs like your stomach make up organ systems, organ systems like your heart, lungs, brain and stomach working together make up an organism, organisms like frogs, dogs, humans, horses, single-celled bacteria, plants, etc., make ecosystems, and ecosystems make environments, and environments make the world!
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