Question: A geologist found several types of seashell fossils in a rock layer. Which is the best conclusion about the rock layer?

  Options: 
 A. The rock was once buried under a swamp.
 B. Water from a river washed across the rock.
 C. The rock was once sediment at the bottom of an ocean.
 D. Migrating animals dropped the seashell into the rock.
Answer: C