You are given a background paragraph that describes one or more causal or qualitative relationships such as a relationship in economics or a scientific law and a story that makes use of the concepts or the relationship described in the provided paragraph. You are also given a question about the story that requires an understanding of the relationship described in the background paragraph and the story. You need to come up with an answer to the given question; the answer will be a span from either the question or the story. In order to correctly answer the given question, you need to understand the relationship mentioned in the background paragraph and should be able to use it to understand that in the story. Your answer can not consist of any word that is not mentioned in any of these: the background paragraph, the story, or the question. You can directly copy and paste a span from the story or the question while answering the given question.

Background Paragraph: The example of someone having a positive experience with a drug is easy to see how drug dependence and the law of effect works. The tolerance for a drug goes up as one continues to use it after having a positive experience with a certain amount the first time.[38] It will take more and more to get that same feeling. This is when the controlled substance in an experiment would have to be modified and the experiment would really begin. The law of work for psychologist B. F. Skinner almost half a century later on the principles of operant conditioning, "a learning process by which the effect, or consequence, of a response influences the future rate of production of that response.[39]. 
Story: The Boat Ballers met to discuss their experiences. They all said they always had a great experience using boat and used the same amount each time. They told how many times they used boat. Les used it 99 times, Osvaldo used it 89 times, Dino used it 79 times, Refugio used it 69 times, Coleman used it 59 times, Margarito used it 49 times, and Alden used it 39 times. 
Question: Who has a higher tolerance for boat: Refugio or Les?
Les.

Background Paragraph: Like water in the ocean, air in the atmosphere exerts pressure that increases with depth. Most gas molecules in the atmosphere are pulled close to Earth’s surface by gravity. As a result, air pressure decreases quickly at lower altitudes and then more slowly at higher altitudes. This is illustrated in Figure below . Air pressure is greatest at sea level, where the depth of the atmosphere is greatest. At higher altitudes, the pressure is less because the depth of the atmosphere is less. For example, on top of Mount Everest, the tallest mountain on Earth, air pressure is only about one-third of the pressure at sea level. At such high altitudes, low air pressure makes it hard to breathe and is dangerous to human health. 
Story: Robert and his brother Timothy are both marathon runners. As marathon running is an endurance event, a key component is being able to replenish your oxygen supply by taking frequent breaths. This week, Robert is running a marathon in Colorado, which has low air pressures. Timothy is  also running a marathon, but he is running in Florida, which has high air pressures. 
Question: Who will have less difficulty in keeping a steady supply of oxygen during their marathon?
Timothy.

Background Paragraph: Naturally occurring uranium is composed almost entirely of two isotopes, uranium-238 (99%) and uranium-235 (1%). It is the uranium-235 that is fissionable (will undergo fission) and therefore, this is the uranium isotope than can be used for fuel in a nuclear reactor. For uranium to be used as fuel, the percent of uranium-235 must be increased to at least 3%. Uranium in which the U-235 content is more than 1% is called enriched uranium. Somehow, the two isotopes must be separated so that enriched uranium is available for use as fuel. Separating the isotope by chemical means (chemical reactions) is not successful because the isotopes have exactly the same chemistry. The only essential difference between U-238 and U-235 is their atomic masses; as a result, the two isotopes are separated by a physical means that takes advantage of the difference in mass. 
Story: John is a nuclear scientist who works at the Fermi lab. Today, in front of him he has four samples, sample A, sample B, sample C, and sample D. Sample A is the naturally occurring uranium. Sample B is enriched uranium. Sample C is the U-238, and sample D is the U-235. He needs to figure out how to best use them. 
Question: Which sample would have lower percentage of uranium-235, sample A or sample B?

sample A.