When Serum Glucose Concentrations Are Decreased, Glucagon Is Released from the Pancreas

When serum glucose concentrations are decreased, glucagon is released from the pancreas and binds to a receptor on hepatic cell membranes. As a result, hepatic glycogen synthesis is inhibited. By which of the following mechanisms is glycogen synthase modified to cause this inhibition?

A) allosteric inhibition by bound AMP
B) binding of intracellular glucose 1-phosphate with a lower K_m
C) dephosphorylation by a glucagon-dependent phosphatase
D) increased amounts of the enzyme due to the CREB protein
E) phosphorylation on serine residues