A scientist has a container filled with carbon dioxide gas. The particles of this gas are far apart, moving quickly, and bouncing off the container walls. Then the particles slow down and collect at the bottom of the container, such that they are close together and vibrating in a rigid structure. What caused this change?

A.
Thermal energy was removed.
B.
The container was opened.
C.
Thermal energy was added.
D.
The particles settled over time.