A group of students was hiking along a riverbank in Louisiana and found a fossil of a shell in the gravel. One student identified the shelled organism as an organism that once lived on the seafloor. ​
Which student best explains how the shelled organism fossil formed over time?

A. Student A states that the organism must have traveled to the river from the ocean and that the fossil formed over the last few years.
B. Student B states that the organism must have traveled to the river from the ocean and that the fossil formed over the last few thousand years.
C. Student C states that the Louisiana area must have once been covered by the sea and that the fossil formed over hundreds of years.
D. Student D states that the Louisiana area must have once been covered by the sea and that the fossil formed over millions of years.