The Figure Below Illustrates How the Nephron in a Human

Multiple Choice

The figure below illustrates how the nephron in a human kidney aids in the removal of toxic by-products of protein metabolism and assists in the regulation of internal water and solute concentrations.
In the region of the nephron labeled "tubule," sodium is

A) leaving the tubule by diffusion.
B) being actively pumped out of the tubule.
C) moving out of the tubule through osmosis.
D) exiting the tubule via secretion.