The Keeling Curve in Figure 25

The Keeling curve in Figure 25.1, below, shows that while CO₂ levels oscillate on an annual basis, overall CO₂ levels increase from year to year. Why has this trend continued in the last 50 years?

A) increasing rates of aerobic respiration associated with the rise in world population
B) decreasing rates of oxygenic photosynthesis by plants and algae
C) clearing of forests to create agricultural land outside the tropics
D) combustion of fossil fuels