Cell theory reveals (　)A. The unity of the structure of living organisms B. The difference between plant cells and animal cells C. Why do cells produce new cells D.

A. One of the main contents of the cell theory is that "animals and plants are composed of cells." This shows the unity of the structure of living organisms. A is correct;

B. Cells The theory does not explain the difference between animal and plant cells. Error B;

C. The cell theory states that new cells can be produced from old cells, but it does not reveal why cells produce new cells. Error C;

D. The process of establishing the cell theory shows that people's understanding of cells is a difficult and tortuous process. D is wrong.

So choose: A.