Cells are the basic building blocks of all living organisms, and they work together in specialized groups to form tissues, which then form organs, and finally organ systems.

Here's why this is the correct answer:

* Cells are the fundamental unit of life: All living things are made up of cells.

* Specialization leads to tissues: Cells within a body system often specialize to perform specific functions. Groups of similar cells working together form tissues (e.g., muscle tissue, nervous tissue).

* Tissues create organs: Different types of tissues work together to form organs (e.g., heart, stomach, brain).

* Organs form organ systems: Organs that work together to perform a common function make up an organ system (e.g., the circulatory system, the digestive system).

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