Task: Question Answering
Input (English): Background Paragraph: New species develop naturally through the process of natural selection . Due to natural selection, organisms with traits that better enable them to adapt to their environment will tend to survive and reproduce in greater numbers. Natural selection causes beneficial heritable traits to become more common in a population and unfavorable heritable traits to become less common. For example, a giraffe’s neck is beneficial because it allows the giraffe to reach leaves high in trees. Natural selection caused this beneficial trait to become more common than short necks. 
Story: There is a creature called a Brotor that has inhabited the planet Glarknon for thousands of years. The Brotor have two distinct traits on them, but each Brotor only has one of these traits. The vast majority of Brotors have fins on their body, but there is a very small segment of the Broto population that has arms instead of fins. 
Question: Which trait has been developed by natural selection?
Output (English): fins.