Concept:Life is organized into increasing levels of complexity, from the smallest building blocks to the largest ecological units.
Explanation:Cells are the basic unit of life.
Similar cells work together to form tissues.
Different tissues combine to form organs.
Organs work together within an organism.
Organisms of the same species form a population.
Different populations interact to form a community.
Communities interact with the non-living environment to form an ecosystem.
Therefore, the sequence from smallest to largest is: cells, tissues, organs, organisms, populations, communities, ecosystems.
Answer:D. Cells, tissues, organs, organisms, populations, communities, ecosystems.