they will stop producing energy. they were formed millions of years ago. they are always available to produce energy. they take millions of years to start producing energy.
I gave my students this multiple choice question: Wind and water are examples of renewable energy sources because

Only one answer is correct among these 4 choices:
- they will stop producing energy.
- they were formed millions of years ago.
- they are always available to produce energy.
- they take millions of years to start producing energy.

Could you tell me which one is correct?

they are always available to produce energy.