Francine is developing a model of the carbon cycle, which involves the movement of carbon among various Earth systems. She begins by drawing the diagram shown below. Carbon cycles between terrestrial and aquatic organisms and atmospheric carbon in the form of CO2 (carbon dioxide). CO2 cycles between gas in the air and dissolved CO2 in ocean water. Carbon in marine sediments and carbonate rocks can be released as carbon dioxide gas by volcanic activity. Carbon in terrestrial and aquatic organisms can become the carbon in fossil fuels such as coal, oil, and natural gas. The carbon in fossil fuels can be converted to carbon dioxide gas. Forest fires can convert carbon compounds in wood to carbon dioxide gas.

Her friend Byron evaluates the model, and suggests that she thicken the arrow leading from fossil fuels to the atmosphere. His reasoning is that fossil fuels are being burned at a much faster rate than natural processes are restoring them.

How does Francine's model show the effect of photosynthesis and cellular respiration on Earth systems?
A. The model shows that their combined effects determine the amount of solid carbon compounds in the geosphere.
B. The model shows that their combined effects determine the level of carbon dioxide that dissolves in the hydrosphere.
C. The model shows that they have identical effects on carbon dioxide levels in the atmosphere.
D. The model shows that they have opposite effects on carbon dioxide levels in the atmosphere.