Here's a problem to solve: Ana is a cross-country runner. After finishing a race she noticed that she felt hot due to the release of thermal energy. Which is the most likely source of the thermal energy?

Among the 4 following options, which is the correct answer?

- A: friction from the air
 
- B: heat being absorbed from the sun
 
- C: mechanical energy being absorbed
 
- D: energy conversions occurring in her body
 