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 am hesitating between 4 options to answer the following question, which option should I choose?
Question: Fossil fuels are considered a nonrenewable resource because they
Possibilities:
- 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.

take a long period of time to form.