About what is the highest level of parts per million of carbon dioxide (ppm) in the air in the 400,000 years before modern time? (In other words, what level of carbon dioxide was never exceeded?)

Question

a. carbon dioxide levels never went over about 250ppm
b. carbon dioxide levels never went over about 275ppm
c. carbon dioxide levels never went over about 300ppm

Answer 1

b. carbon dioxide levels never went over about 275ppm