Increasing heat causes thermal expansion, and liquid propane is used to heat hot air balloons. Given these facts, how is liquid propane used in hot air ballooning among 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      