How Do Scientists "Know" the Atmospheric Levels of Carbon Dioxide

How do scientists "know" the atmospheric levels of carbon dioxide that existed on Earth hundreds or thousands of years ago?

A) They deduce it from the carbon dioxide content of fossil fuels.
B) They measure it from bubbles of gas trapped in Antarctic ice.
C) They estimate it from the relative numbers of carbon-based life forms that existed over time.
D) They measure it from erupting volcanoes, bubbling hot springs, and seeping hydrothermal vents.