What determines which sex will result in offspring of birds and mammals?

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

Which would be the most useful source of evidence to support mcneill's contention?

2. which is the last step in the scientific method?