The graph shows atmospheric carbon dioxide levels over time. Which two observations are most useful for explaining the trends in the data? Select two answer choices.
A. Photosynthesis and cellular respiration have opposite effects on carbon dioxide levels.
B. Carbon is stored in solid form in shells, rocks, and sediments.
C. Atmospheric carbon dioxide may dissolve in ocean water.
D. The burning of fossil fuels adds carbon dioxide to the atmosphere.
E. Activities that produce or take up carbon dioxide vary with the seasons.