What Are the α- and β-Subunits of the Sodium-Potassium ATP

What are the α- and β-subunits of the sodium-potassium ATP responsible for?

A) The α-subunit is responsible for binding sodium ions. The β-subunit is responsible for binding potassium ions.
B) The α-subunit is responsible for the enzymatic activity of the pump and contains all the substrate-binding sites. The β-subunit has several extracellular glycosylation sites and is necessary for pump function.
C) The α-subunit is responsible for binding sodium and potassium ions. The β-subunit is the substrate of phosphorylation.
D) The α-subunit is not necessary for pump function. The β-subunit contains binding sites for sodium, potassium and ATP.
E) The sodium-potassium ATPase consists of a single polypeptide chain without a β-subunit.