Answer: Choice D
Cells are the primary unit of structure in living organisms.
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Step-by-step explanation:
According to cell theory, cells are the building blocks of any living organism. This pertains to unicellular organisms (aka creatures that are one cell only) or multicellular organisms. This is why choice D is the answer.
Choice A is false because animal cells range in size. The largest animal cell is the ostrich egg that is about 5.1 inches across. In contrast, a neuron in the human body is about 100 microns across.
Choice B is false because plant cells have different shapes depending on the role they play in the plant itself.
Choice C is false. The number of cells varies even within the same species of a plant or animal. This is because the number is randomly determined and changes all the time. Old cells die and new ones are created constantly.