Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Longitudinal waves, like sound waves, have compression and rarefaction zones. The compression zones are where, for example in a sound wave traveling through air, the air molecules are densely packed. The rarefaction zones are areas where the air molecules are loosely packed, like a vacuum zone. Us teachers will often draw longitudinal waves as transverse waves, but remember the difference. Answer: compression zones
What is the opposite of rarefaction zones, where air molecules in waves are loosely packed?