The Great Oxygenation Event Was a Time in the Middle

Multiple Choice

The Great Oxygenation Event was a time in the middle Proterozoic when

A) organisms that produce oxygen first originated.
B) oxygen began to accumulate as a gas in the atmosphere.
C) the oxygen concentration in the atmosphere reached its present concentration of
21 percent.
D) excessively high concentrations of oxygen in the atmosphere caused a mass extinction.