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.

[EX Q]: Background Paragraph: A dilution effect is seen when animals living in a group "dilute" their risk of attack, each individual being just one of many in the group. George C. Williams and W.D. Hamilton proposed that group living evolved because it provides benefits to the individual rather than to the group as a whole, which becomes more conspicuous as it becomes larger. One common example is the shoaling of fish. Experiments provide direct evidence for the decrease in individual attack rate seen with group living, for example in Camargue horses in Southern France. The horse-fly often attacks these horses, sucking blood and carrying diseases. When the flies are most numerous, the horses gather in large groups, and individuals are indeed attacked less frequently. Water striders are insects that live on the surface of fresh water, and are attacked from beneath by predatory fish. Experiments varying the group size of the water striders showed that the attack rate per individual water strider decreases as group size increases. 
Story: Kenya's Masai Mara wildebeest migration is one of the greatest spectacles in the natural world. Every year thousands of wildebeests cross the Mara river. John went to see the migration last year. He noticed that the wildebeests crossed the the river in a very large group. He noted that as case A. His guide told him that is not the case in other times of the year. The wildebeests cross the river in smaller groups in other times. John noted that as case B. The guide added that in both cases on thing is constant, they are attacked by the lions. 
Question: Which case would see the dilution effect working better, case A or case B?
[EX A]: case A.

[EX Q]: Background Paragraph: Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more places for chemical weathering to occur ( Figure below ). Let’s say you wanted to make some hot chocolate on a cold day. It would be hard to get a big chunk of chocolate to dissolve in your milk or hot water. Maybe you could make hot chocolate from some smaller pieces like chocolate chips, but it is much easier to add a powder to your milk. This is because the smaller the pieces are, the more surface area they have. Smaller pieces dissolve more easily. 
Story: Rocks commonly found in Beijing are often found spread out in smaller pieces, whereas rocks in Montreal typically remain large in size and don't break apart as much. Both Beijing and Montreal are expected to experience bouts of acid rain in the upcoming weeks. 
Question: Which area's rocks will have higher rates of chemical weathering caused by the acid rain?
[EX A]: Beijing.

[EX Q]: Background Paragraph: Any unused energy in food, whether it comes from carbohydrates, proteins, or lipids, is stored in the body as fat. An extra 3,500 Calories of energy results in the storage of almost half a kilogram (1 pound) of stored body fat. People who consistently consume more food energy then they need may become obese. Obesity occurs when the body mass index is 30.0 kg/m 2 or greater. Body mass index (BMI) is an estimate of the fat content of the body. It is calculated by dividing a person’s weight (in kilograms) by the square of the person’s height (in meters). Obesity increases the risk of health problems such as type 2 diabetes and hypertension. 
Story: Two neighboring villages were living mostly off the land. Boar village was full of fat people, because they would overeat lots of animal products. Leap village was inhabited by vegetarians and vegans.Their food consumption was moderate. 
Question: Which village had more people with increased risk of hypertension?
[EX A]:
Boar.