It is b because it just is
Answer:
The answer is Covection.
Explanation:
Convection current occurs where hot / warm air rises and cold air drops and replace it.
Over time, as oxygen levels decrease, CO2 levels increase. Respiration is correct as it uses oxygen as a reactant and releases CO2 as a byproduct.
The random movement of molecules across a concentration gradient until the difference in concentration between two regions is near zero.
Im thinking it's active transport :) hope it helps.