Here's a problem to solve: What makes solar energy different from most other energy that people use?

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

- A: Solar energy involves the most dangerous equipment.
 
- B: Solar energy requires the most complicated technology.
 
- C: The supply of solar energy will not change for billions of years.
 
- D: The supply of solar energy causes it to be the cheapest kind of energy.
 