When a ligand attaches to a receptor, which of the following occurs first? (Consider a ligand to be a hormone or a cell messenger)
A) There is a change in shape of the ligand which causes the signaling cascade
B) There is a change in shape of the receptor which triggers the stopping of the signaling cascade
C) There is a change in shape of the receptor which triggers the beginning of the signaling cascade
D) There is a change in shape of the receptor which causes the ligand to be released