The surfaces absorb energy from the air. Friction changes energy of motion to heat energy. Moving objects produce heat energy. Energy is transferred between the two surfaces.
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: Why do two surfaces get warm when they are rubbed together?
Possibilities:
- The surfaces absorb energy from the air.
- Friction changes energy of motion to heat energy.
- Moving objects produce heat energy.
- Energy is transferred between the two surfaces.

Friction changes energy of motion to heat energy.