The Intracellular Calcium Ion Concentration of Ventricular Muscle Cells Averages

The intracellular calcium ion concentration of ventricular muscle cells averages 10-4 mmol/L during diastole. The calcium ion concentration in transverse tubules (T tubules) averages 2.5 mmol/L at rest. A protein transporter on the membrane of the T tubule exchanges sodium for calcium. The transporter uses the transmembrane sodium gradient to fuel the exchange. Which of the following transport mechanisms best describes this type of transporter?

A) Facilitated diffusion
B) Primary active transport
C) Secondary active co-transport
D) Secondary active counter-transport
E) Simple diffusion