Malik draws a model to explain changes of state for helium as shown. From left to right, the illustration shows three boxes labeled 'solid', 'liquid' and 'gas'. An arrow labeled 'Step X' points from gas to liquid. Another arrow points from liquid to solid.

Step X describes how helium gas could become a liquid. Which of the following could be the label for Step X?

A
Thermal energy is added and molecular motion increases.

B
Thermal energy is added and molecular motion decreases.

C
Thermal energy is removed and molecular motion increases.

D
Thermal energy is removed and molecular motion decreases.