The graph illustrates atmospheric carbon dioxide concentration in a country from 1968 to 2000. Carbon dioxide concentration changes from year to year. What is the best explanation for these variations?
- Aincreased rate of photosynthesis in the summer months of each year
- Bincreased rate of respiration in the winter months of each year
- Chigher rainfall in the summer months of each year
- Dincreasing yearly temperatures due to global warming