Glycolysis - Medical Science

Q1:

The active form of glycogen phosphorylase is phosphorylated, while the dephosphorylation of which active form occurs?

A Glycogen synthase

B Glycogen semisynthase

C Glycogen hydrolase

D Glycogen dehydrogenase

ANS:A - Glycogen synthase

I haven't understood this. Please explain me in detail.