What is a prerequisite (requirement) for counter-current exchange to function?

What is a prerequisite (requirement) for counter-current exchange to function?

Select all that apply.

Select one or more:

a. gills

b. respiratory pigments in blood

c. unidirectional flow of the outside medium (air or water)

d. concurrent flow of blood in capillary beds and external oxygenated medium (water or air)

e. bulk flow (mass flow) of blood and external medium (air or water)

f. tidal flow of external medium (air or water)