The binding of a signaling molecule (often called a ligand) to a receptor is required to "turn on" the signaling pathway. The reversal of this process, the unbinding of the signal, is requried to turn off the pathway. What usually causes the signla molecule to leave its binding site on the receptor protein?

(a) diffusion of the signal molecule into the cell
(b) a decrease in the concentration of the signal molecule outside the cell
(c) The activity of phosphatase enzymes, removing phosphate from proteins in the signal transduction pathway
(d) the breakdown of second messengers in the cell