Context: This passage describes a chemical reaction. Read the passage. Then, follow the instructions below.
Plants make glucose, a sugar they can use for food, through a process called photosynthesis. During photosynthesis, plants use energy from sunlight to make glucose from water and carbon dioxide. In the process of making glucose, plants also create most of the oxygen in the air we breathe.
Complete the sentence.
In this chemical reaction, carbon dioxide is a ().
A. product
B. reactant
Select the correct answer from the provided options and reply with the letter associated with it.

B