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