When We Think About the Carbon Cycle and Human Activities

When we think about the carbon cycle and human activities, it is important to differentiate between facts and hypotheses. Which of the following can be considered a fact?

A) The amount of carbon dioxide in the atmosphere has increased since 1950.
B) Increasing atmospheric carbon dioxide will cause mean global temperature to increase by 2 degrees Celsius over the next century.
C) The burning of fossil fuels contributes substantially to the ongoing rise of atmospheric CO₂.
D) In the past, atmospheric CO₂ levels reached levels higher than those observed today.