Context: This passage describes a chemical reaction. Read the passage. Then, follow the instructions below.
The Space Shuttle program sent astronauts on 135 missions using a fleet of five shuttles between 1981 and 2011. The engines of each shuttle, like other modern rocket engines, used liquid hydrogen as fuel. When liquid hydrogen combines with liquid oxygen, an enormous amount of energy is released, along with water vapor. The shuttles used this energy to launch into space.
Complete the sentence.
In this chemical reaction, oxygen is a ().
A. reactant
B. product
Identify the correct option from the choices provided and respond with the letter of the correct option.

A