Context: This passage describes a chemical reaction. Read the passage. Then, follow the instructions below.
Like people on Earth, astronauts living on the International Space Station need oxygen to survive. Since there is no oxygen in space for the astronauts to breathe, they make oxygen using a process called electrolysis. This process uses electricity to break down water into hydrogen gas and oxygen gas. The hydrogen gas can be removed from the space station or recycled, and the oxygen gas is released into the air that the astronauts breathe.
Complete the sentence.
In this chemical reaction, hydrogen is a ().
A. product
B. reactant
From the given choices, choose the correct answer and respond with the letter of that choice.

A