Why did Charles Keeling's carbon dioxide graphs show an up and down pattern each year?

W) More carbon dioxide was generated by heating processes during the winter

X) The air is denser in the winter, causing carbon dioxide to become diluted

Y) Carbon dioxide is absorbed during the growth season and released through decay during the fall and winter

Z) Warmer summer continents release more carbon dioxide than during colder winters
Y) CARBON DIOXIDE IS ABSORBED DURING THE GROWTH SEASON AND RELEASED THROUGH DECAY DURING THE FALL AND WINTER