How is the variation in life produced?

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

What best explains the inability for life to exist in earth early atmosphere

Which best describes a gene? a. a sister chromatid b. a chromosome c. a tetrad d. a piece of a chromosome