What is one way coral reefs are important to the health of the biosphere?

What did not occur in the proterozoic eon a)the oxygen levels increase greatly in the atmosphere,b)the emergence of the eukarya , c)muti cella’s eukaryotic organisms began to diversify,d)anaerobic bacteria kept thriving with more oxygen

I’m stuck on this question cuz i’m stupid sksksksk

About how much of the cell mass is watet