I'm pretty sure it's Cells
I believe the biosphere is fragile, because over time all of the pollution us humans have made; If its from air pulloution, acid rain or even water pollution. the toxic acids over time have made the biosphere and the ozon layer weaker and more thin. which causes the biosphere to become weaker.
It should be B because when people breath in it is oxygen and when we breath out it is carbon dioxide
Answer:
B. refraction
Explanation: hope this helps!