When Examining the Keeling Curve in the Accompanying Graph, One

When examining the Keeling Curve in the accompanying graph, one notices a "zigzag" upward- moving curve. What do these small up and down movements indicate?

A) latitudinal differences in CO₂ concentrations
B) weather- related factors affecting CO₂
C) uncertainty in the CO₂ conditions
D) seasonal cycles in CO₂ concentrations
E) O₂ versus CO₂ concentrations