Volcanic activity on the early Earth released large volumes of gases, forming an early atmosphere that was highly analogous to the modern atmosphere of Mars. Which gas was the dominant component of this early atmosphere?

• Oxygen
• Methane
• Carbon dioxide
• Nitrogen

Approximately what fraction of the Earth's modern dry atmosphere consists of nitrogen ($N_2$)?

• $\frac{1}{5}$
• $\frac{1}{2}$
• $\frac{4}{5}$
• $\frac{9}{10}$

Over billions of years, the concentration of carbon dioxide in the Earth's atmosphere decreased dramatically from its initially high levels. Identify two primary geological or biological pathways that were responsible for this massive sink of atmospheric carbon dioxide.

• The decay of organic matter by aerobic bacteria
• The absorption of carbon dioxide by oceans, leading to the precipitation of insoluble metal carbonates (such as limestone)
• The expansion of early photosynthetic organisms, such as cyanobacteria and algae, which fixed carbon dioxide into organic compounds
• The intense release of carbon dioxide from deep-sea hydrothermal vents
• The thermal decomposition of carbonate rocks in the Earth's mantle