When glucagon binds to its receptor on the membrane of the liver cell, intracellular cAMP is produced and activates protein kinase A, which in turn activates glycogen phosphorylase, the enzyme that breaks down glycogen to glucose-1-phosphate. In relation to this signaling cascade, which of the following correctly describes cAMP?
A Signal distributor
B Cellular response
C Second messenger
D Signal transducer
E Effector protein