Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
A refrigerator must do work to reverse the normal direction of thermal energy flow. Work involves the use of force to move something, and doing work takes energy. In a refrigerator, the energy is usually provided by electricity. You can read in detail in the Figure below how a refrigerator does its work. For an animation of how a refrigerator works, go to this URL: http://www. chemistry. wustl. edu/~edudev/LabTutorials/CourseTutorials/LabTutorials/Thermochem/fridge_movie. html.


Question: A refrigerator must do work to reverse the normal direction of what?
thermal energy flow
thermal energy flow.