Question: An unusual type of fossil clam is found in rock layers high in the Swiss Alps. The same type of fossil clam is also found in the Rocky Mountains of North America. From this, scientists conclude that

  Options: 
 A. glaciers carried the fossils up the mountains
 B. the Rocky Mountains and the Swiss Alps are both volcanic in origin
 C. clams once lived in mountains, but have since evolved into sea-dwelling creatures
 D. the layers of rocks in which the fossils were found are from the same geologic age
Answer: D