Why are carbon dioxide concentrations expected to increase? a. carbon dioxide concentrations are expected to increase, because fossil fuel burning is expected to increase. b. carbon dioxide concentrations are expected to increase, because of an increase in agriculture. c. carbon dioxide concentrations are expected to increase, because more land will have to be cleared for increasing populations and agricultural use. d. all of the above

The answer to this is D.

All of the above activities of human may cause an increase in carbon dioxide concentration. First of all, as fossil fuels are still the main source of energy, and it is very cheap and convenient, it is more likely to be used more in the future since renewable energy is not as common. But the burning of fossil fuels (also combustion), can greatly increase the carbon dioxide in the air, as carbon dioxide is one of the products after the reaction. 

Agriculture can also cause an increase in carbon dioxide concentrations since agriculture also includes the rearing of animals (not only growing of crops), where livestock may consume more oxygen and release carbon dioxide during respiration. In addition, people may cut down trees (deforestation) for the land for agriculture, which means fewer trees can absorb carbon dioxide. 

Last but not least, again, as more land is cleared for the increasing population and agricultural use, fewer trees can absorb carbon dioxide through photosynthesis and more carbon dioxide would be found in the air. 

Therefore, the answer should be D, all of above.