Answer the following question given this paragraph: 

The digestive system of birds is unique, with a gizzard that contains swallowed stones for grinding food. Birds do not have teeth. What do you think the stones do? They help them digest their food. Defining characteristics of modern birds also include:.


Q: What does the gizzard in birds contain that allows them to grind food?

 Choices:

- acid

- bile

- teeth

- swallowed stones

A:
swallowed stones
