The Keeling Graph of Carbon Dioxide Concentrations Form a Pattern

The Keeling graph of Carbon Dioxide concentrations form a pattern that goes up and down each year. What is the cause of this oscillating pattern?

A) More combustion of wood and fossil fuels occur in the winter than the summer contributing to the increased CO₂.
B) In the summer of Hawaii, much of the snow melt on Mauna Loa and CO₂ is released.
C) During the northern hemisphere summer, there are so many trees, that they reduce the amount of CO₂ substantially, but when northern winter occurs, there are not enough trees in the southern hemisphere to use all of the inputs of CO₂ in the atmosphere.
D) During the southern hemisphere summer, there are so many trees, that they reduce the amount of CO₂ substantially, but when southern winter occurs, there are not enough trees in the northern hemisphere to use all of the inputs of CO₂ in the atmosphere.