Context: This passage describes a chemical reaction. Read the passage. Then, follow the instructions below.
Many watches are powered by small, flat batteries called button cells. One common type of button cell contains the metal zinc. When zinc in the battery combines with oxygen in the air, zinc oxide forms. This process generates the electricity that powers the watch.
Complete the sentence.
In this chemical reaction, zinc oxide is a ().
A. product
B. reactant
Answer with the option's letter from the given choices directly.
Answer: A