Answer the following question given this paragraph: 

The enzyme pepsin works only in the presence of acid. A strong acid is secreted into the stomach. It provides the acid that pepsin needs. You will learn about acids in the next chapter, "Chemistry of Solutions. " Vinegar and lemon juice are two common acids.


Q: The enzyme pepsin works only in the presence of what?

 Choices:

- carbohydrates

- sodium

- oxygen

- acid

A:
acid
