Atmospheric CO2 is distinctly higher in the winter than the summer, and this variability is much stronger in the northern hemisphere than the southern hemisphere. Which of the following is responsible for this seasonal variability? Choose one.
A. There is more industrial output of CO2 in the northern hemisphere
B. Forest burning occurs primarily in the summer
C. Northern hemisphere land plants have well defined seasons of growth and respiration
D. There is more industrial output of CO2 in one season