When a ligand attaches to a receptor, which of the following occurs first? (Consider a ligand to be a hormone or a cell messenger)

AThere is a change in shape of the ligand which causes the signaling cascade

BThere is a change in shape of the receptor which triggers the stopping of the signaling cascade

CThere is a change in shape of the receptor which triggers the beginning of the signaling cascade

DThere is a change in shape of the receptor which causes the ligand to be released