Answer the following question given this paragraph: 

Fluoride ion is widely used in water supplies to help prevent tooth decay. Chloride is an important component in ion balance in blood. Iodide ion is needed by the thyroid gland to make the hormone thyroxine.


Q: Fluoride ion is widely used in water supplies to help prevent what?

 Choices:

- rust

- bacteria

- chlorine taste

- tooth decay

A:
tooth decay
