-Cells are composed of water, inorganic ions, and carbon-containing (organic) molecules. Water is the most abundant molecule in cells, accounting for 70% or more of total cell mass.
-Large quantities of water molecules constantly move across cell membranes by simple diffusion, often facilitated by movement through membrane proteins, including aquaporins. In general, net movement of water into or out of cells is negligible.
The cell membrane helps to control what enters and leaves a cell. The substance which accounts for two-thirds of a cells mass is water. That makes the answer D.
