The major ATP-powered pump responsible for maintaining ion gradients across the plasma membrane of mammalian cells is:

A)the calmodulin-activated plasma membrane Ca²⁺ ATPase.
B)the sarcoplasmic reticulum Ca²⁺ ATPase.
C)the vacuolar F-class proton pump.
D)the plasma-membrane Na⁺/K⁺ ATPase.