Question: A geologist found fossils of horse bones in different rock layers of a canyon. The fossils in the lower levels show shorter bones than those in the upper-level fossils. Which conclusion is based on the geologist's observations?

  Options: 
 A. Horse's reproduce using two parents.
 B. Horse's became larger over time.
 C. Smaller Horse's went extinct, and then larger Horse's formed.
 D. Smaller Horse's are better suited to the environment than larger Horse's.
Answer: B