Here's a problem to solve: How do scientists know that some mountains were once at the bottom of an ocean?

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

- A: Freshwater rivers flow to the ocean.
 
- B: Saltwater fish are found in some mountain streams.
 
- C: Dinosaur bones have been discovered in the mountains.
 
- D: Marine fossils have been found on the peaks of some mountains.
 