Signaling Molecules Can Be "Tagged" by Attaching Glow-In-The-Dark Molecules to Them

Signaling molecules can be "tagged" by attaching glow-in-the-dark molecules to them. After adding tagged signaling molecules to the fluid around several cells, you observe the cells with a microscope designed to see glow-in-the-dark molecules. After a few minutes, you notice that the plasma membrane of some of the cells begins to glow. This observation indicates that some cells

A) have more dense plasma membranes than others.
B) have receptors for the signaling molecule.
C) have no active carrier proteins for the signaling molecules.
D) block the signaling molecules by repelling them.