A team of scientists uses radioactive carbon to label glucose molecules. The radioactivity does not affect the chemical or biological properties of the glucose molecules. In an experiment, the team places red blood cells in an isotonic solution. Then they add radioactive glucose to the solution. They observe that levels of radioactivity increase in the interior of the cell, indicating that the cell has taken up the radioactive glucose from the solution. Which observation best supports the conclusion that the glucose entered the red blood cells by means of passive transport? A. Glucose can pass through specific proteins in the cell membrane. B. Glucose can pass through the lipid bilayer of cell membrane. C. The cell expended energy in the form of ATP during the experiment. D. The cell did not expend energy in the form of ATP during the experiment.