If Plants Consume CO₂ During Photosynthesis, Why Hasn't All the Atmospheric

If plants consume CO₂ during photosynthesis, why hasn't all the atmospheric CO₂ been used up?

A) Photosynthesis also produces CO₂ as a product.
B) Respiration produces CO₂ as a product.
C) Both photosynthesis and respiration produce CO₂ as products.
D) Plants actually don't use CO₂ during photosynthesis; they use O₂.
E) Photosynthesis and respiration use one another's products as reactants.