Schleiden and Schwann, are known best for bringing up the cell theory. Although there are other scientists such as Robert Hook who are known to have contributed to the cell theory but Schleiden and Schwann are considered to be the pioneer of the cell theory.
Matthias Schleiden was a botanist which means that his expertise was in studying plants. Theodor Schwann was a zoologist which means that his expertise was in studying animals. Hence, Theodor Schwann was the scientist who studies animals and so option, D is correct.
The respiratory and circulatory systems bring oxygen and nutrients to the cells. The respiratory and circulatory systems work together to maintain homeostasis. The respiratory system moves gases into and out of the blood.
