What is the main cause of carbon dioxide build up in the atmosphere?

What is the main cause of carbon dioxide build up in the atmosphere?
 
  a. burning fossil fuels
  b. deforestation
  c. droughts
  d. flooding
  e. respiration due to overpopulation

Question 2

The Hubbard Brook Watershed Project illustrated that
 
  a. deforestation can alleviate drought conditions.
  b. deforestation can result in polluted streams.
  c. deforestation results in calcium retention in soil.
  d. primary consumers are the most important part of that energy pyramid.
  e. trees absorb too much water and are not useful in a watershed area.