Fact 1: If gas is heated then that gas will expand.

Fact 2: If a material is heated, it expands and the density decreases.

Given the two facts above, answer the question "When does a gas's density decrease?" with the following options: 
- Time and energy
 - cool off
 - Dehydration
 - air cools
 - It becomes dormant
 - Evaporation
 - Dilution
 - When it's heated


 

        When it's heated 