Answer the following question given this paragraph: 

Sound is a form of energy that travels in waves. Sound waves cannot travel through empty space, but they can travel through gases. Gases in the air allow us to hear most of the sounds in our world. Because of air, you can hear birds singing, horns tooting, and friends laughing. Without the atmosphere, the world would be a silent, eerie place.


Q: What helps sound travel in waves?

 Choices:

- molecules

- speaker placements

- temperatures

- gases

A:
gases
