Answer the following question given this paragraph: 

Compounds like sodium chloride form structures called crystals. A crystal is a rigid framework of many ions locked together in a repeating pattern. Ions are electrically charged forms of atoms. You can see a crystal of sodium chloride in the Figure below . It is made up of many sodium and chloride ions.


Q: Compounds like sodium chloride form structures called what?

 Choices:

- ions

- microorganisms

- hydrocarbons

- crystals

A:
crystals
