You are presented with the question "What changes a solid into a liquid using heat energy?" and the following answer choices: 
- electricity
 - It gets heated up
 - boiling
 - aqueous solution
 - a phase change
 - a hydraulic system
 - heating liquids
 - Water expanding

Now knowing that melting means changing from a solid into a liquid by adding heat energy and melting is a phase change, choose the best answer.


 

     a phase change    