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Diffusion of potassium in all regions of the filter occurs during countercurrent flow because the generation of diffusion gradient between water and the blood.

In counter current, the blood in the capillaries flowing in the opposite direction to the flow of water which creates a steep diffusion gradient  which results in rapid transfer of potassium into the blood so we can conclude that diffusion occur in all regions of the filter due to countercurrent flow of both liquids.

Learn more about countercurrent here: link

Learn more: link

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.

currents is the correct answer.