That is a prerequisite (requirement) for counter-current exchange to function? Select all that apply. Select one or more:
a. tidal flow of external medium (air or water)
b. unidirectional flow of the outside medium (air or water)
c. concurrent flow of blood in capillary beds and external oxygenated medium (water or air)
d. bulk flow (mass flow) of blood and external medium (air or water)
e. gills
f. respiratory pigments in blood

D, E, and F

Explanation:

The requirements for counter current exchange are gills, bulk flow of blood and water and air and the blood pigment. Also, the fluids should flow in close proximity and flow in the opposite directions.

d

Explanation:

only requirement is the bulk movement of blood and air or water, everything else is structural

c

explanation:

the circled areas are referred to   as sunspots.   the sunspots are cooler than the   surrounding area.

sunspots are temporary phenomena on the photosphere of the sun that appear visibly as dark spots compared to surrounding regions. they correspond to concentrations of magnetic field flux that inhibit convection and result in reduced surface temperature compared to the surrounding photosphere.