Please choose the correct answer to the 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?

Answer1: Horse's reproduce using two parents. Answer2: Horse's became larger over time. Answer3: Smaller Horse's went extinct, and then larger Horse's formed. Answer4: Smaller Horse's are better suited to the environment than larger Horse's.

Answer format: answer1/answer2/answer3/answer4
Answer: answer2