You Do an Experiment in Which You Increase the Concentration

You do an experiment in which you increase the concentration of phosphate in a solution and measure the uptake by plant cells. You find that once the concentration of phosphate reaches 1 gram per liter adding more phosphate does not increase uptake any further. From this you would conclude that phosphate is taken up by ______ .

A) exocytosis.
B) facilitated diffusion.
C) active transport.
D) endocytosis.
E) osmosis.