Context: This passage describes a chemical reaction. Read the passage. Then, follow the instructions below.
When the Statue of Liberty in New York City was built in the late 1800s, it was reddish-brown. Today, a green layer, called a patina, covers the statue's surface. This layer formed over time as copper in the statue combined with oxygen and other gases in the air to form the patina.
Complete the sentence.
In this chemical reaction, copper is a ().
A. reactant
B. product
From the given choices, choose the correct answer and respond with the letter of that choice.

A