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.