The diagram is a model of one way that materials move across the cell membrane. Oxygen molecules are represented by two red circles. Carbon dioxide molecules are represented by a gray circle with two red circles attached. The left-hand side of the diagram shows the beginning concentrations of the two types of molecules. The right-hand side shows how the molecules have moved over time. A. They move to an area with a lower oxygen concentration.

B. They become transport proteins in the cell membrane.

C. They merge with the cell membrane to form vacuoles.

D. They move to an area with a higher oxygen concentration.