Answer the following question given this paragraph: 

Remember that a phase change depends on the direction of the heat transfer. If heat transfers in, solids become liquids, and liquids become solids at the melting and boiling points, respectively. If heat transfers out, liquids solidify, and gases condense into liquids.


Q: What type of direction does a phase change depend on?

 Choices:

- weight transfer

- power transfer

- humidity transfer

- heat transfer

A:
heat transfer
