One substance has no membrane transporters to move it but can diffuse freely through open leak channels if there is a concentration gradient. Initially, this substance's concentrations in the filtrate and extracellular fluid are equal. Later, however, the active transport of Na+ and other solutes creates a gradient by removing water from the lumen of the tubule where it is located. What substance is this?
A) calcium
B) glucose
C) urea
D) glucose and calcium
E) glucose, calcium, and urea