have turned into solid rock. take a long period of time to form. are expensive to remove from rock. are located deep within the Earth's crust.
I gave my students this multiple choice question: Fossil fuels are considered a nonrenewable resource because they

Only one answer is correct among these 4 choices:
- have turned into solid rock.
- take a long period of time to form.
- are expensive to remove from rock.
- are located deep within the Earth's crust.

Could you tell me which one is correct?

take a long period of time to form.