You are presented with the question "How is liquid propane used in hot air ballooning?" and the following answer choices: 
- 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

Now knowing that increasing heat causes thermal expansion and liquid propane is used to heat hot air balloons, choose the best answer.


 

   to expand the balloon      