The rock was once buried under a swamp. Water from a river washed across the rock. The rock was once sediment at the bottom of an ocean. Migrating animals dropped the seashell into the rock.
I gave my students this multiple choice question: A geologist found several types of seashell fossils in a rock layer. Which is the best conclusion about the rock layer?

Only one answer is correct among these 4 choices:
- The rock was once buried under a swamp.
- Water from a river washed across the rock.
- The rock was once sediment at the bottom of an ocean.
- Migrating animals dropped the seashell into the rock.

Could you tell me which one is correct?

The rock was once sediment at the bottom of an ocean.