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 sides shows how the molecules have moved over time. which sentence best explains the movement of oxygen molecules over time? a. they move to an area with a higher oxygen concentration b. they merge with the cell membrane to form vacuoles c. they move to an area with lower oxygen concentration d. they become transport proteins in the cell membrane