How can scientists know some mountains were once at the bottom of an ocean?

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

Answer: (A)

When scientists find marine fossils on the peaks of mountains, it shows those rocks formed in a sea. Fossils like shells, corals, and other organisms lived in ocean water, so their presence indicates a marine environment when the rock formed. Over long timescales, movements of Earth's plates can lift that seabed upward, creating mountains. Dating the rocks and recognizing the marine fossils together confirm that the mountains grew from ocean-floor material that was later pushed up above sea level.

The other options don’t provide the same clear evidence. A river flowing to the ocean describes current processes, not past environments. Finding saltwater fish in mountain streams could happen if pathways connect to the sea, but it doesn’t show that the mountains themselves formed from ancient ocean floor. Dinosaur bones in mountains point to land-dwelling animals and don’t by themselves indicate an ancient ocean bottom.