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单项选择题

The hormone glucagons(胰高血糖素) stimulates the breakdown of stored glycogen in liver and muscle cells by the following enzymes. Which is the first enzyme that must be activated ( )

A.Protein kinase A
B.Phosphorylase kinase
C.Glycogen phosphorylase
D.Protein phosphatase
E. Protein phosphatase inhibitor-1