Photosynthesis Consists of Two Chemical Reactions, the Light-Dependent and the Light-Independent

Photosynthesis consists of two chemical reactions, the light-dependent and the light-independent reactions. What is the cellular function of CO₂ in the photosynthesis reaction?

A) Carrier of high-energy electrons
B) The source of carbon used to build carbohydrates
C) The output of photosynthesis that is used to build carbohydrates
D) The source of electrons in the light-dependent reaction