Why Are Phosphorylation and Dephosphorylation Reactions Commonly Used to Regulate

Why are phosphorylation and dephosphorylation reactions commonly used to regulate signal transduction pathways?

A) Phosphate groups are efficient second messengers
B) Protein kinases and phosphatases are abundant in most cells
C) The hydrolysis of bound GTP generates GDP,which can change the activity of proteins bound to GDP
D) The addition or removal of a phosphate group can expose or hide potential binding sites in proteins or change protein activity