What is sodium-potassium pump ATPase?

  A.  There is a net flux of sodium from intracellular spaces into luminal spaces through sodium-potassium pump ATPase spanning proteins located in the luminal membranes of epithelial cells in the medullary collecting duct of the kidney.

  B.  There is a net flux of sodium from intracellular spaces into extracellular spaces through sodium-potassium pump ATPase spanning proteins located in the plasma membranes of toe motor neurons.

  C.  There is a net flux of sodium from cytosol near troponin molecules into the internal spaces of the sarcoplasmic reticulum through sodium-potassium pump ATPase spanning proteins located in the sarcoplasmic reticulum membranes of diaphragm muscles.