were formed from different fossils took different amounts of time to form formed at different times formed by different methods
I gave my students this multiple choice question: Limestone is a sedimentary rock and marble is a metamorphic rock. Even though limestone and marble have the same chemical makeup, they are classified as different rocks because they

Only one answer is correct among these 4 choices:
- were formed from different fossils
- took different amounts of time to form
- formed at different times
- formed by different methods

Could you tell me which one is correct?

formed by different methods