Scientists have correlated average global temperature with atmospheric carbon dioxide levels for the last 160,000 years using ice cores from Antarctica. How is the carbon dioxide level determined?A. by measuring the CO2 concentration in trapped gases in the ice layers
B. by measuring the 18O/16O ratio in the CO2 molecules
C. by extrapolating the CO2 level from the temperature of the ice
D. by extrapolating the CO2 level from the average global temperature