Cardiac Glycosides Such as Ouabain Stimulate Cardiac Contraction

Cardiac glycosides such as ouabain stimulate cardiac contraction. The molecular basis of the effect of these compounds is believed to result from which of the following?

A) They act as monovalent cation ionophores.
B) They cause an increase in the fluidity of the plasma membrane.
C) They inhibit the efflux of calcium ions from the cell.
D) They inhibit the Na⁺,K⁺-ATPase enzyme.
E) They become intercalated into the plasma membrane, rendering it leaky.