Here's a problem to solve: A scientist discovered a fossil of an ocean plant in the rocks of a desert. What does the discovery of this fossil most likely tell the scientist?

Among the 4 following options, which is the correct answer?

- A: The fossils were formed at the top of a mountain.
 
- B: The fossils were formed during a rainstorm.
 
- C: The area had many volcanic eruptions.
 
- D: The area was once covered by water.
 