Fact 1: Increasing heat causes thermal expansion.

Fact 2: Liquid propane is used to heat hot air balloons.

Given the two facts above, answer the question "How is liquid propane used in hot air ballooning?" with the following options: 
- lowered energy cost
 - The area was once underwater
 - to expand the balloon
 - energy of moving objects
 - activation energy
 - hydrogen and oxygen
 - It remains dormant
 - to move people


 

   to expand the balloon      